Musnad Ahmad 1153:1239

Shababah ibn Sawwar narrated to us, Shu'bah informed us, from Nu'aym ibn Abi Hind, from Abu Wa'il, from Masruq, from Aisha, she said: "The Messenger of Allah (peace and blessings of Allah be upon him) prayed behind Abu Bakr while sitting, during his illness in which he died."