Sahih Muslim Book 4, Hadith Number 1608.

Chapter : Permissibility of observing Nafl (voluntary prayer) standing or sitting and observing some part of it standing or sitting.

Abu Salama asked ‘Aisha about the prayer of the Messenger of Allah (may peace be upon him) She said: He observed thirteen rak’ahs (in the night prayer). He observed eight rak’ahs and would then observe Witr and then observe two rak’ahs sitting, and when he wanted to bow he stood up and then bowed down, and then observed two rak’ahs in between the Adhan and Iqama of the dawn prayer.