Musannaf Ibn Abi Shayba 3:1350

Abdullah ibn al-Mubarak and Waki’ narrated to us, from Ibn ‘Awn, from Raja’ ibn Haywah, from Mahmud ibn al-Rabi’, from al-Sunabihi, who said: I prayed Maghrib with Abu Bakr, and I drew near to him until my clothes touched his clothes, or my hand touched his clothes—Ibn al-Mubarak was in doubt. He recited in the third rak'ah the Opening of the Book (Fatihat al-Kitab) and said: "Our Lord, let not our hearts deviate after You have guided us."