Musnad Ahmad 1153:2166

Azhar ibn al-Qasim narrated to us, Hisham narrated to us, from Qatadah, from Zurarah ibn Awfa, from Sa'd ibn Hisham, from Aisha: "That the Prophet of Allah, may Allah bless him and grant him peace, used to, when he performed Witr, pray two rak'ahs while he was sitting."