Fiqh-us-Sunnah, Volume 2: Actions which invalidate the Salah
Volume2, Page 95a: Intentionally eating or drinking invalidates the salah
Ibn al-Munzhir says: "The people of knowledge agree that if oneintentionally eats or drinks during a fard salah, he is to repeat the salah.The same is the case with nawafil according to the majority of scholars as whatinvalidates an obligatory (fard) prayer also invalidates a voluntary (nafl)prayer."
Volume2, Page 95b: Speaking intentionally about something unrelated to the salahinvalidates the salah
Intentionally speaking during the salah, if it is not beneficial to thesalah, invalidates the salah.
Zaid ibn Arqam relates: "We used to talk while we were in salah and aperson would speak to the person next to him until the verse was revealed: 'Andstand before Allah in devout obedience' and we were then commanded to observesilence during the salah." This is related by the group.
Ibn Mas'ud reports: "We used to greet the Messenger of Allah while hewas in salah and he would respond to our greeting. When we returned fromAbyssinia, we greeted him [during prayer] but he did not respond to oursalutation. We said to him: 'O Messenger of Allah, we used to greet you whileyou were in salah and you used to respond to us!' He then said: 'Prayer demandsone's complete attention.'" This is related by al-Bukhari and Muslim.
If one is ignorant of this ruling or speaks due to the fact that he hasforgotten this ruling, his salah will still be valid
Mu'awiyyah ibn alHakam said: "I was praying behind the Messenger ofAllah and someone in the congregation sneezed. I said [to him]: 'May Allah havemercy upon you.' The people then stared at me, showing their disapproval of myact. I said: 'Woe to me, why do you stare at me so?' They started to striketheir hands on their thighs and when I saw that they wanted me to becomesilent, I was angered but said nothing. When the Messenger of Allah finishedthe prayer - and may my father and mother be ransomed for him, I found noteacher better than him either before or after him - he did not scold, beat, orrevile me but he simply said: 'Talking to others is not seemly during thesalah, for the salah is for glorifying Allah, extolling His Greatness, andreciting the Qur'an.'" This is related by Ahmad, Muslim, Abu Dawud, andan-Nasa'i. Mu'awiyyah ibn al-Hakam spoke out of ignorance of this ruling andthe Prophet sallallahu alehi wasallam did not order him to repeat his salah.
Talking [if it is a reminder as to the incompleteness of the salah], doesnot nullify the salah as can be seen in the following hadith. Abu Hurairahsays: "The Messenger of Allah led us in either the noon or after-noonprayers and he made the taslim after praying just two rak'at. Zhul Yadain saidto the Prophet: 'O Messenger of Allah, has the salah been shortened or have youforgotten [part of it]?' The Prophet sallallahu alehi wassalam said: 'It hasnot been shortened, nor did I forget any part of it." He said: 'Yes, OMessenger of Allah, you did forget.' Thereupon the Prophet asked (the people):'Is Zhul Yadain correct in what he says?' The people said: 'He is correct, youoffered only two rak'at.' Then, the Prophet prayed the two remaining rak'at andmade the taslim, said the takbir and performed the sajdah, sat and made thetakbir and performed the sajdah again, and finally said the takbir and satagain." This is related by al-Bukhari and Muslim.
The Maliki school allows talking during the prayer if it is done for anygood of the salah as long as it does not become a common practice and (is done)only when saying subhanallah fails to alert the imam to correct his mistake.Al-Auza'i's comments are: "Whoever intentionally speaks during the salah,seeking some benefit to the salah, does not invalidate his salah." He saidthat if a person recites aloud in the 'asr and someone behind him says:"It is the 'asr," (i.e., the recital is not to be aloud) then thelatter person would not invalidate his salah.
Volume2, Page 96: Intentionally making many motions
The scholars differ over what exactly constitutes a few motions and whatconstitutes many motions. Some say that one makes many motions when, if seenfrom behind, one would be certain that he was not performing salah, andanything less than that amount is considered only a few motions. Some say thatit is any act or string of actions which would make others believe that theperson is not praying.
An-Nawawi says: "If a person performs a lot of actions that are notpart of the salah, he invalidates his salah, and, on this point, there is nodifference of opinion. If the acts are few, then they do not invalidate thesalah and, on this point, there also is no difference of opinion. This is theexact position. However, there does exist a difference of opinion over whatexactly constitutes a few actions and many actions, [and there exist fouropinions on this point..." He says that the fourth opinion is the correctand most popular opinion. The fourth opinion is that the exact definitions oftoo much and too little are determined by generally accepted standards. One isnot harmed in his salah by common acts such as nodding in reply to asalutation, taking off one's shoes, raising the headdress and putting it backin place, putting on or taking off a light garment, carrying or holding a smallchild, preventing someone from passing in front of the person in prayer,covering one's spittle in one's clothing and similar other actions. As for theother acts, those which are considered to constitute many actions (e.g., takingmany consecutive steps, performing actions repeatedly) they invalidate theprayer. An-Nawawi also says: "The scholars are in agreement that manyactions invalidate the prayer if they are performed consecutively [i.e., oneafter another]. If one separates the actions, for instance, taking a step andthen stopping for a while, then taking another step or two, and then anothertwo steps, after a pause (though a short one) between them, then the salah willnot be harmed, even if he (in this manner should take a hundred or more steps.There is no difference of opinion on this point. As for light actions," hecontinues, "such as, moving one's finger in glorifying Allah or initching, and so forth., these do not invalidate the prayer according to the well-known,authentic opinion, even when they are done repeatedly and consecutively, butthey are disliked." AshShaf'i, in a statement concerning it, says:"Even if one counts the verses on one's fingers, it would not invalidateone's salah, but it is best to avoid [such an act]."
Volume2, Page 97: Intentionally leaving out an essential act or condition of theprayer without any valid excuse for doing so
Al-Bukhari and Muslim record that the Prophet sallallahu alehi wasallam tolda bedouin who had not performed his salah well: "Return and pray for youhave not prayed." (This hadith was mentioned earlier.)
Ibn Rushd writes: "There is an agreement that if one prays and he isnot in a state of purity, it is obligatory for him to repeat the prayer, [thatis true if the act was done] intentionally or out of forgetfulness. Similarly,one who prays without facing the qiblah, intentionally or due to forgetfulness,[must repeat the salah]. In general, if any of the conditions for thecorrectness of the salah are absent, it becomes obligatory to repeat the salah."
Volume2, Page 98: Smiling or laughing during the salah
Ibn al-Munzhir records that there is a consensus of opinion that laughing(during the salah) invalidates the prayer. An-Nawawi says: "This is thecase if one laughs aloud, and produces sound. Most of the scholars say thatthere is no problem with smiling. If one is overcome by laughter and cannotcontrol it, his salah will not become invalid if it is of minor nature. If itis a hearty laughter, it will invalidate the salah. Custom would determinewhether it is a major or a minor laughter."