Source: SSahīh al-Bukhārī, 19 – THE BOOK OF SSALĀT-UT-TAHAJJUD (التهجد), vol.2/p.147-148
(11) CHAPTER. The waking up of the Prophet صلى الله عليه وسلم from his sleep for the night prayer and what (how much) was cancelled from the night prayer
And the Statement of Allāh ta’ālā: “O you wrapped in garments (i.e. Prophet Muhammad صلى الله عليه وسلم ! Stand (to pray) all night, except a little. Half of it, or a little less than that, or a little more; and recite the Qur-ān (aloud) in a slow (pleasant tone and) style. Verily, We shall send down to you a weighty Word (i.e. obligations, laws). Verily, the rising by night (for Tahajjud prayer) is very hard and most potent and good for governing (oneself), and most suitable for (understanding) the Word (of Allāh). Verily, there is for you by day prolonged occupation with ordinary duties.” [73:1-7]
And the Statement of Allāh: “…He Knows that you are unable to pray the whole night, so He has turned to you (in mercy). So, recite you of the Qur-ān as much as may be easy for you. He knows that there will be some among you sick, others travelling through the land, seeking of Allāh Bounty, yet others fighting in the Cause of Allāh. So recite as much of the Qur-ān as may be easy (for you), and perform as-SSalāt (Iqāmat-as-SSalāt) and give Zakāt, and lend to Allāh, a goodly loan, and whatever good you send before you for yourselves, (i.e. non-obligatory acts of worship: prayers, charity, fasting, Hajj and ‘Umra, etcetera), you will certainly find it with Allāh, better and greater in reward…” [73:20]
 Narrated Anas bin Mālik radiAllāhu ‘anhu:
Sometimes the Messenger of Allāh صلى الله عليه وسلم would not observe SSaum (fast) (for so many days) that we thought that he would not observe SSaum (fast) that month and he sometimes used to observe SSaum (fast) (for so many days) that we thought he would not leave observing SSaum (fast) throughout that month and [as regards his SSalāt (prayers) and sleep at night], when you wanted to see him offering SSalāt (prayer) at night, you could see him offering SSalāt (prayer) and when you wanted to see him sleeping, you could see him sleeping.