It is narrated from Hazrat Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) used to perform Witr with nine rak'ahs. Then, when his age increased and his blessed body became heavy, the Prophet (peace and blessings be upon him) started performing Witr with seven rak'ahs.
Hadith Referenceمسند احمد / مسند النساء / 25889
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد اختلف فيه على الأعمش