Musnad Ahmad 27:1437

Ishaq ibn Isa narrated to us, Layth narrated to us, Yazid ibn Abdullah ibn al-Had narrated to me, from Abdullah ibn Dinar, from Abdullah ibn Umar, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "The most righteous of righteousness is for a man to maintain ties with the people his father loved after he has passed away."