Al-Muzzammil 73:20

Assuredly your Lord knows that you stand vigil less than two thirds of the night, or [at times] a half of it or a third of it (if read wa-nisfihi wa-thuluthihi, then these constitute a supplement to thuluthay, ‘two thirds’; if read wa-nisfahu wa-thuluthahu, then a supplement to adnā, ‘less than’) — his keeping vigil in the way mentioned is in accordance with what was enjoined on him at the beginning of this sūra — along with a group of those with you (wa-tā’ifatun mina’lladhīna ma‘aka constitutes a supplement to the subject [of the verb] taqūmu, ‘you stand vigil’, but it may also constitute, although it is not certain, a separating clause). The keeping vigil by some of his companions in this way indicates their emulation of him. Some of them could not tell how much of the night they had spent in prayer and how much of it had remained, and would therefore keep vigil all night as a precaution; and so they used to keep vigil [in this way] for a whole year or more with their feet swollen, until God alleviated matters for them. He, exalted be He, says: and God keeps measures, He keeps count of, the night and the day. He knows that (an: softened in place of the hardened form, its subject having been omitted, that is to say, annahu) you will not be able to keep count of it, that is, the [length of the] night, so that you may perform the vigil at the time in which it is required unless you stay up all night, which is hard on you, and so He has relented to you, making you revert to what is easier. So recite as much as is feasible of the Qur’ān, during prayer, by performing as much prayer as is feasible. He knows that (an: softened in place of the hardened form, that is to say, annahu) some of you will be sick, while others will be travelling in the land, seeking the bounty of God, seeking of His provision through commerce and otherwise, and others will be fighting in the way of God: for each of the three groups mentioned the keeping of nightly vigil is hard, and so God has alleviated things for them by [enjoining on them] what is feasible. Later this was abrogated by the five [daily] prayers. So recite as much as is feasible of it (as [explained] above) and perform, obligatory, prayer and pay alms and lend God, by expending in the way of good in addition to what is due from your wealth [as alms], a goodly loan, out of the goodness of [your] hearts. For whatever good you send ahead for [the sake of] your souls, you will find that, with God, it will be better, than what you left behind (huwa khayran: huwa is for separation; what follows it [khayran, ‘better’] is comparable to a definite, even if it is not [actually] so, on account of the impossibility of it being given [the al- of] definition) and greater in terms of reward. And seek forgiveness from God; assuredly God is Forgiving, Merciful, to believers.