Sahih Al Bukhari Hadith Number 1148

Narrated `Aisha: I did not see the Prophet (peace be upon him) reciting (the Qur’an) 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.

19  Prayer at Night (Tahajjud)
Hadith number 1148
Volume no 2
Chapter: 16 (The Salat (prayer) of the Prophet (peace be upon him) at night in Ramadan and (in) other months)