Abu Dawud Book 2, Hadith Number 1358.

Chapter : On the number of rakahs of the prayer at night.

Narated By ‘Aisha, Ummul Mu’minin : Al-Aswad ibn Yazid said that he entered upon ‘Aisha and asked her about the prayer of the Apostle of Allah (PBUH) during the night. She said: He used to pray thirteen rak’ahs during the night. Then he began to pray eleven rak’ahs and left two rak’ahs. When he died, he would pray nine rak’ahs during the night. His last prayer during the night was witr.