Sahih Bukhari
Prayer at Night (Tahajjud)
Volume: 002 / Book Number: 021 / Hadith Number: 249
Narrated 'Aisha:
I did not see the Prophet reciting (the Quran) in the night prayer while sitting except when he became old; when he used to recite while sitting, and when thirty or forty verses remained from the Sura, he would get up and recite them and then bow.