Musnad Ahmad 1018:45

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, 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 after ‘Ali, may Allah be pleased with him, had been killed, a man from the Azd tribe—dark-skinned and tall—stood up and said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, placing him [al-Hasan] in his lap, saying: 'Whoever loves me, let him love him; let the witness inform the absent.' And were it not for the determination of the Messenger of Allah, may Allah bless him and grant him peace, I would not have told you."