It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to pray nine rak'ahs of witr. Then, when he (peace and blessings be upon him) grew older and gained weight, he began praying seven rak'ahs of witr, and he would pray two rak'ahs with two prostrations while seated, in which he recited Surah al-Zalzalah, Surah al-Kafirun and Surah al-Ikhlas.
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8033