Musannaf Ibn Abi Shayba 3:4243

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Abu Hurrah informed us, he said: Al-Hasan narrated to us, from Sa'd ibn Hisham, from 'Aishah, she said: When the Messenger of Allah (may Allah bless him and grant him peace) would stand at night to pray, he would begin his prayer with two light rak'ahs.