Khatib Baghdadi, Tarikh-e-Baghdad, V.2, page 79; Ibn Maghazli, Manaqib, page 34; Dailami, Firduas Al-Akhbar, V.1, page 41, hadith 93; Hakim Naishapuri, Al-Mustadrak, V.3, page 147; Ibn Asaker, Tarikh Madinah-Damishq, V.42, page 40. And it has been narrated from Omar who said: Abu Ubaidah, Abu Bakr, a group of Ansar and myself were together when the Messenger of Allah (s.a.w.a.) placed his hand on Ali’s shoulders and said, “O Ali! You are the first of the believers (in faith) and the first to accept Islam and you are for me as Aaron to Moses.”

Refer to Ibn-Dimashqi, Jawaher al-Matalib, V. 1, page 38; Ibn Abd al-Barr, Al-Isti’ab, V.3, page 1091; Ibid, At-Tamhid, V.2, page 305; Muttaqi Hindi, Kanz al-Ummal, V.11, page 616.