Musnad Ahmad 1:41

'Ali ibn 'Ayyash narrated to us, Al-Walid ibn Muslim narrated to us, he said: And Yazid ibn Sa'id ibn Dhi 'Uswan al-'Ansi informed me, from 'Abd al-Malik ibn 'Umayr al-Lakhmi, from Rafi' al-Ta'i, the companion of Abu Bakr in the Battle of al-Salasil, he said: And I asked him about what was said regarding their pledge of allegiance. He said—while he was narrating to him about what the Ansar had spoken, and what he had spoken to them, and what 'Umar ibn al-Khattab had spoken to the Ansar, and what he had reminded them of regarding my leading them in prayer by the command of the Messenger of Allah, may Allah bless him and grant him peace, during his illness—: "So they pledged allegiance to me for that, and I accepted it from them, and I feared that there would be a tribulation, and that there would be apostasy after it."