Musnad Ahmad 462:19

Haywah ibn Shurayh narrated to us, Baqiyyah narrated to us, Buhayr ibn Sa’d narrated to us, from Khalid ibn Ma’dan, who said: Al-Miqdam ibn Ma’di Karib and ‘Amr ibn al-Aswad arrived as a delegation to Mu’awiyah. Mu’awiyah said to al-Miqdam: "Do you know that al-Hasan ibn ‘Ali has passed away?" Al-Miqdam recited "Inna lillahi wa inna ilayhi raji'un" (Indeed we belong to Allah and indeed to Him we shall return). Mu’awiyah said to him: "Do you consider it a calamity?" He replied: "And why should I not consider it a calamity, when the Messenger of Allah, peace and blessings of Allah be upon him, used to place him on his lap and say: 'This is from me, and Husayn is from ‘Ali'?"