Musnad Ahmad 435:21

Hashim ibn al-Qasim narrated to us, Hariz narrated to us, from Abd al-Rahman ibn Abi Awf al-Jurashi, from Mu'awiyah, who said: I saw the Messenger of Allah, may Allah bless him and grant him peace, sucking his tongue—or he said: his lip—meaning al-Hasan ibn Ali, may the prayers of Allah be upon him. And a tongue or two lips that the Messenger of Allah, may Allah bless him and grant him peace, sucked will never be punished.