Musannaf Ibn Abi Shayba 27:550

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from 'Abdullah ibn al-Harith, from Zuhayr ibn al-Aqmar, who said: While al-Hasan ibn 'Ali was delivering a sermon, a man from [the tribe of] Azd stood up—he was dark-skinned and tall—and said: "I saw the Messenger of Allah (may Allah bless him and grant him peace) placing him [al-Hasan] on his hips, saying: 'Whoever loves me, let him love him. Let the witness inform the absent.'"