Musnad Ahmad 5:332

Shuja' ibn al-Walid narrated to us, he said: Khalaf ibn Hawshab mentioned, from Abu Ishaq, from 'Abd Khayr, from 'Ali, who said: "The Prophet, may Allah bless him and grant him peace, preceded, and Abu Bakr prayed, and 'Umar made it three. Then a tribulation struck us—or befell us—and Allah pardons whom He wills."