Abu Ghalib narrates from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to pray seven rak'ahs of witr. When his (peace and blessings be upon him) body became heavy, he (peace and blessings be upon him) would pray three witr and after that offer two rak'ahs sitting, reciting in them "When the earth is shaken" [Surah al-Zalzalah: 1] and "Say: O disbelievers" [Surah al-Kafirun: 1].
Imam al-Bukhari (may Allah have mercy on him) used to say: Umarah bin Zadhan is sometimes inconsistent (mudtarib) in his hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4827