Thabit al-Bunani narrates from Anas bin Malik (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) used to pray nine rak'ahs of witr. When he (peace and blessings be upon him) grew older and his body became heavier, he (peace and blessings be upon him) would pray seven witr, and after that pray two rak'ahs sitting, reciting in them Surah al-Rahman and Surah al-Waqi'ah.
(b) Anas (may Allah be pleased with him) says: We recite, from the short surahs of the Mufassal, "When the earth is shaken" [Surah al-Zalzalah: 1] and "Say: O disbelievers" [Surah al-Kafirun: 1].
Umarah bin Zadhan differed from all the other narrators concerning what the Prophet (peace and blessings be upon him) recited in these two rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4826