Musannaf Ibn Abi Shayba 27:584

'Abd al-Rahim ibn Sulayman narrated to us, from 'Amir, who said: Al-'Abbas entered upon the Prophet, may Allah bless him and grant him peace, and he did not see anyone with him. His son said to him, "I indeed saw a man with him." So al-'Abbas said, "O Messenger of Allah, your cousin claims that he saw a man with you." He said, "Yes, by the One who sent down the Book upon you." He said, "That was Gabriel."