Sahih Muslim Book 4, Hadith Number 1616.

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

‘Aisha reported: When the Apostle of Allah (may peace be upon him) had prayed the two rak’ahs (Sunan) of the dawn prayer, he would talk to me if I was awake, otherwise he would lie down.