Al-Aswad bin Yazid came to Aishah (may Allah be pleased with her) and asked about the night prayer of the Prophet (peace and blessings be upon him), and she said: "The night prayer of the Prophet (peace and blessings be upon him) used to be thirteen rak'ahs. Then he (peace and blessings be upon him) prayed eleven rak'ahs and left two rak'ahs. Then, until his death, he (peace and blessings be upon him) prayed nine rak'ahs at night. His (peace and blessings be upon him) last prayer of the night was the witr."
Muhammad bin Ishaq bin Khuzaymah differed from him, narrating it from Mu'ammal bin Hisham through Abu Ishaq from Masruq; but the narration of Abu Dawud is more correct, as shown by the narration of Ammar bin Ruzayq mentioned earlier.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4832