Musnad Ahmad 1297:0

Narrated by Umm ‘Umarah bint Ka‘b: The Messenger of Allah (peace and blessings of Allah be upon him) entered upon me, and I brought him food. He said to me, "Eat." I said, "I am fasting." The Messenger of Allah (peace and blessings of Allah be upon him) said, "If food is eaten in the presence of a fasting person, the angels pray for blessings upon him until they finish."