Musnad Ahmad 1166:24

Bahz narrated to us, Hammad ibn Salamah narrated to us, Simak ibn Harb narrated to us, from Harun, the son of the daughter of Umm Hani—or the son of the son of Umm Hani—from Umm Hani, that the Messenger of Allah (peace and blessings of Allah be upon him) drank a drink, then he handed it to her so she could drink. She said: "I am fasting, but I disliked rejecting your leftovers." He said—meaning: "If it is a makeup for Ramadan, then make up a day in its place. And if it is a voluntary fast, then if you wish, make it up, and if you wish, do not make it up."