In Surat al-Muzzammil (73), the verse: "Keep vigil at night but a little, a half thereof" (2-3) has been abrogated by the following verse: "He knoweth that ye count it not, and turneth unto you in mercy. Recite then of the Qur'an that which is easy for you" (v.20). The phrase "the vigil of the night" (nashi'at al-layl) means the early hours of the night. They (the companions) would pray (the tahajjud prayer) in the early hours of the night. He (Ibn Abbas) says: It is advisable to offer the prayer at night (tahajjud), prescribed by Allah for you (in the early hours of the night). This is because when a person sleeps, he does not know when he will awake. The words "speech more certain" (aqwamu qilan) means that this time is more suitable for the understanding of the Qur'an. He says: The verse: "Lo, thou hast by day a chain of business" (v.7) means engagement for long periods (in the day's work).  

فِي الْمُزَّمِّلِ { قُمِ اللَّيْلَ إِلاَّ قَلِيلاً * نِصْفَهُ } نَسَخَتْهَا الآيَةُ الَّتِي فِيهَا { عَلِمَ أَنْ لَنْ تُحْصُوهُ فَتَابَ عَلَيْكُمْ فَاقْرَءُوا مَا تَيَسَّرَ مِنَ الْقُرْآنِ } وَنَاشِئَةُ اللَّيْلِ أَوَّلُهُ وَكَانَتْ صَلاَتُهُمْ لأَوَّلِ اللَّيْلِ يَقُولُ هُوَ أَجْدَرُ أَنْ تُحْصُوا مَا فَرَضَ اللَّهُ عَلَيْكُمْ مِنْ قِيَامِ اللَّيْلِ وَذَلِكَ أَنَّ الإِنْسَانَ إِذَا نَامَ لَمْ يَدْرِ مَتَى يَسْتَيْقِظُ وَقَوْلُهُ { أَقْوَمُ قِيلاً } هُوَ أَجْدَرُ أَنْ يُفْقَهَ فِي الْقُرْآنِ وَقَوْلُهُ { إِنَّ لَكَ فِي النَّهَارِ سَبْحًا طَوِيلاً } يَقُولُ فَرَاغًا طَوِيلاً  

When the opening verses of Surah Al-muzammil was revealed, the Companions would pray as long as they would pray during Ramadan until its last verses were revealed. The period between the revelation of its opening and the last verses was one year.  

لَمَّا نَزَلَتْ أَوَّلُ الْمُزَّمِّلِ كَانُوا يَقُومُونَ نَحْوًا مِنْ قِيَامِهِمْ فِي شَهْرِ رَمَضَانَ حَتَّى نَزَلَ آخِرُهَا وَكَانَ بَيْنَ أَوَّلِهَا وَآخِرِهَا سَنَةٌ