"Indeed, your Lord knows that you stand [in prayer] less than"—meaning less than—"two-thirds of the night," up to His saying, "He has known that you will not be able to enumerate it [the night], so He has forgiven you." Qatada's interpretation: The obligation of night prayer was at the beginning of this Surah: "O you who wrap yourself, arise [to pray] the night, except for a little—half of it or subtract from it a little or add to it." So the Messenger of Allah ﷺ and his companions stood for a year until their feet became swollen. Allah held back its conclusion in the heaven for twelve months, then He revealed, "Indeed, your Lord knows that you stand [in prayer] less than two-thirds of the night, and half of it, and a third of it," and some read it as "and a third of it," up to His saying: "So recite what is easy from it. And establish prayer and give zakah"—these are two obligatory duties—"and loan Allah a goodly loan." Al-Hasan's interpretation: This is regarding voluntary deeds. "And whatever good you put forward for yourselves, you will find it with Allah—it is better."
﴿إِنَّ رَبَّكَ يَعْلَمُ أَنَّكَ تَقُومُ أدنى﴾ أَقَلَّ ﴿مِنْ ثُلُثَيِ اللَّيْلِ﴾ إِلَى قَوْلِهِ ﴿عَلِمَ أَنْ لَنْ تُحْصُوهُ﴾ اللَّيْل ﴿فَتَابَ عَلَيْكُم﴾ تَفْسِير (ل ٣٧٧) قَتَادَةَ: كَانَ الْفَرْضُ قِيَامَ اللَّيْلِ فِي أوّل هَذِه السُّورَة ﴿يَا أَيهَا الْمُزَّمِّلُ قُمِ اللَّيْلَ إِلا قَلِيلا نِصْفَهُ أَوِ انْقُصْ مِنْهُ قَلِيلا أَو زد عَلَيْهِ﴾ فَقَامَ رَسُولَ اللَّهِ ﷺ وَأَصْحَابُهُ حَوْلاً حَتَّى انْتَفَخَتْ أَقْدَامُهُمْ؛ وَأَمْسَكَ اللَّهُ خَاتِمَتَهَا فِي السَّمَاءِ اثْنَي عَشَرَ شَهْرًا ثُمَّ أَنْزَلَ ﴿إِنَّ رَبَّكَ يَعْلَمُ أَنَّكَ تَقُومُ أَدْنَى مِنْ ثُلُثَيِ اللَّيْلِ وَنِصْفَهُ وَثلثه﴾ وبعضعم يقْرؤهَا ﴿وَثلثه﴾ إِلَى قَوْلِهِ: ﴿فَاقْرَءُوا مَا تَيَسَّرَ مِنْهُ وَأَقِيمُوا الصَّلاةَ وَآتُوا الزَّكَاةَ﴾ فَرِيضَتَانِ وَاجِبَتَانِ، فَصَارَ قِيَامُ اللَّيْلِ تطوُّعًا ﴿وَأَقْرِضُوا اللَّهَ قَرْضًا حَسَنًا﴾ تَفْسِيرُ الْحَسَنِ: هَذَا فِي التطوَّع ﴿وَمَا تُقَدِّمُوا لأَنْفُسِكُمْ مِنْ خَيْرٍ تَجِدُوهُ عِنْدَ اللَّهِ هُوَ خَيْرًا﴾.