Musnad Ahmad 1153:1238

Bakr ibn Isa narrated to us, he said: I heard Shu'bah ibn al-Hajjaj narrating from Nu'aym ibn Abi Hind, from Abu Wa'il, from Masruq, from Aisha: "That Abu Bakr prayed with the people while the Messenger of Allah, may Allah bless him and grant him peace, was in the row."