Musnad Ahmad 5:543

Waki’ narrated to us, from Sufyan, from Abu Hashim ibn Kathir, from Qays al-Kharifi, from Ali, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) preceded, and Abu Bakr prayed, and Umar made it a third. Then a tribulation struck us, and it is as Allah wills."