It is narrated from Hazrat Abu Umamah (may Allah be pleased with him) that in the beginning, the Noble Prophet (peace and blessings be upon him) used to perform nine rak'ahs of Witr. Later, when the blessed body of the Noble Prophet (peace and blessings be upon him) became heavy, he started making Witr with seven rak'ahs. And the Noble Prophet (peace and blessings be upon him) used to perform two rak'ahs sitting after Witr, and in them he would recite Surah Zalzalah and Surah Kafiroon.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 22313
Hadith Gradingحکم دارالسلام:صحيح لغيره دون تعيين قراءة النبىﷺ فى الركعتين بعد الوتر، وهذا إسناد حسن فى المتابعات والشواهد من أجل أبى غالب وعمارة بن زاذان