Musnad Ahmad 32:1086

Abu al-Yaman narrated to us, he said: Shu'ayb informed us, from al-Zuhri, he said: Anas ibn Malik informed me—and he was with the Prophet, may God bless him and grant him peace, and served him and accompanied him—that Abu Bakr used to lead them in prayer during the illness of the Prophet, may God bless him and grant him peace, in which he passed away. Until it was Monday, then he mentioned the hadith.