Musnad Ahmad 1297:1

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Habib, a man from the Ansar, from a freedwoman of theirs who is called Layla, who narrates from my grandmother, and she is Umm ‘Amarah bint Ka‘b, that the Messenger of Allah (may Allah bless him and grant him peace) entered upon her, and she brought food near to him. He said to her: "Eat." She said: "I am fasting." He said: "Indeed, the angels invoke blessings upon the fasting person when food is eaten in their presence, until they finish."