Musnad Abu Ya'la al-Mawsili 42:79

Muhammad ibn 'Abbad al-Makki narrated to us, Abu Sa'id narrated to us, from Sadaqah ibn al-Rabi', from 'Umarah ibn Ghaziyyah, from 'Abd al-Rahman ibn Abi Sa'id, from his father, who said: We were at the house of the Prophet, may Allah bless him and grant him peace, among a group of the Emigrants (Muhajirun) and the Helpers (Ansar). He came out to us and said, "Shall I not inform you of your best ones?" They said, "Yes." He said, "Your best ones are those who are loyal and pure. Indeed, Allah loves the hidden, pious one." He said, and 'Ali ibn Abi Talib passed by, so he said, "The truth is with this one, the truth is with this one."