Tahajjud and Qiyam-ul-Layl (Hadith No. 922)

Volume 2, Book 21, Number 223:
Narrated 'Aisha:
Allah's Apostle used to offer eleven Rakat and that was his prayer. He used to prolong the prostration to such an extent that one could recite fifty verses (of the Quran) before he would lift his head. He used to pray two Rakat (Sunna) before the Fajr prayer and then used to lie down on his right side till the call-maker came and informed him about the prayer.

Qiyaam al-layl means spending the night, or part of it, even if it is only one hour, in prayer, reading Qur’aan, remembering Allah (dhikr) and other acts of worship. It is not stipulated that it should take up most of the night. 
It says in Maraaqi al-Falaah: What is meant by qiyaam is spending most of the night in worship, or it was said: spending one hour of it, in reading Qur'aan, listening to hadeeth, glorifying Allah (tasbeeh) or sending blessings upon the Prophet (blessings and peace of Allah be upon him). End quote. 
Al-Mawsoo‘ah al-Fiqhiyyah al-Kuwaitiyyah, 34/117. 
Tahajjud means specifically praying at night, and some scholars limited it to prayers that are offered at night after sleeping. 
Al-Hajjaaj ibn ‘Amr al-Ansaari (may Allah be pleased with him) said:
One of you thinks that if he gets up at night and prays until morning comes that he has done tahajjud. But in fact tahajjud means praying after sleeping, then praying after sleeping. That is how the Messenger of Allah (blessings and peace of Allah upon him) prayed. Al-Haafiz Ibn Hajar said in al-Talkhees al-Habeer (2/35): Its isnaad is hasan; it includes Abu Saalih, the scribe of al-Layth, and it is somewhat weak. It was also narrated by al-Tabaraani, whose isnaad includes Ibn Luhay‘ah, whose report is supported by the one that came before it.
Thus it becomes clear that qiyaam al-layl is more general than tashahhud, because it includes prayer and other actions, and it includes prayer before and after sleeping. 
But tahajjud is exclusively praying, and there are two opinions concerning it. The first is that it means praying at any time of the night, which is the view of the majority of fuqaha’. 
The second is that it is prayer after sleeping. See: al-Mawsoo‘ah al-Fiqhiyyah, 2/232.

[Taken from IslamQA]
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