Musnad Ahmad 28:105

Hajjaj and Abu al-Nadr narrated to us, they said: Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Abu al-Khayr, from Abdullah ibn Amr, that a man asked the Prophet, peace and blessings of Allah be upon him: "Which deeds are best?" He said: "To feed food, and to offer the greeting of peace to those you know and those you do not know."