Sahih Muslim Book 4, Hadith Number 1589.

Chapter : The virtue of Sunan prayers before and after the Fard Rak’ahs and their number.

‘Aisha reported: I did not see the Messenger of Allah (may peace be upon him) reciting (the Qur’an) in the night prayer in a sitting position, till he grew old and then he recited (it) in a sitting position, but when thirty or forty verses were left out of the Surah, he would then stand up, recite them and then bowed.