Musnad Ahmad 25:1683

Sulayman ibn Dawud narrated to us, Abu 'Awanah narrated to us, from Abu Balj, from 'Amr ibn Maymun, from Ibn 'Abbas, who said: "The first to pray with the Prophet, may Allah bless him and grant him peace, after Khadijah was 'Ali." And he said once: "He embraced Islam."