Surah 17. Al-Israa, Ayah 79

Lit., "as a deed beyond that which is incumbent on thee" (nafilatan laka) - i.e., in addition to the five obligatory prayers. Hence, the above is not an injunction but a recommendation, although the Prophet himself invariably spent the greater part of the night in prayer.

 This refers to the time when the Prophet (ﷺ) will make intercession (shafâ’ah) on the Day of Judgment.

This is held to be addressed specially to the holy Prophet who usually prayed more than the five canonical prayers. The Tahajjud was a prayer after midnight, in the small watches of the morning.

To the Prophet was to be assigned in the Hereafter the highest Post of Honour and Glory-the Maqam Mahmud, implying his excellence above all other Prophets. The immediate reference may be to the hope that the Makkan persecution will soon be over and the glorious work in Madinah will begin.