Musnad Abu Ya'la al-Mawsili 111:41

Hassan ibn Thabit recited verses to the Prophet, peace and blessings of Allah be upon him, and said: "I bear witness, by the permission of Allah, that Muhammad is the Messenger of the One who is above the heavens, from on high. And that Abu Yahya and Yahya, both of them, have deeds in their religion that are accepted. And that the brother of al-Ahqaf, when he stood among them, spoke by the essence of Allah among them and acted justly." The Prophet, peace and blessings of Allah be upon him, said: "And I [bear witness to that as well]."