Pondering over the Great Creation of Allah, The passing away of Life of the World, the Horrors of the Day of Requital and Laxity of One's Nafs
Allah, the Exalted, says:
"I exhort you to one (thing) only: that you stand up for Allah's sake in pairs and singly, - and reflect (within yourselves the life history of the Prophet (PBUH))". (34:46)
"Verily! in the creation of the heavens and the earth, and in the alternation of night and day, there are indeed signs for men of understanding. Those who remember Allah (always, and in prayers) standing, sitting, and lying down on their sides, and think deeply about the creation of the heavens and the earth, (saying): 'Our Rubb! You have not created (all) this without purpose, glory to You! (Exalted be You above all that they associate with You as partners)". (3:190,191)
"Do they not look at the camels, how they are created? And at the heaven, how it is raised? And at the mountains, how they are rooted (and fixed firm)? And at the earth, how it is spread out? So remind them (O Muhammad (PBUH)), you are only a one who reminds". (88:17-21)
"Have they not travelled through the earth, and seen..". (47:10)
Ayat of the Qur'an and Ahadith on the subject are many. See for example Hadith No. 66 mentioned before.