Musnad Ahmad 1272:7

Safwan ibn Isa narrated to us, he said: Abu Yunus al-Qushayri Hatim ibn Abi Saghirah narrated to us, from Simak ibn Harb, from Abu Salih, from Umm Hani: That the Prophet, peace and blessings of Allah be upon him, entered upon her on the day of the Conquest, and she brought him a drink. He drank from it, then some of it remained, so he handed it to her and she drank it. Then she said: "O Messenger of Allah, I have done something and I do not know if it is agreeable to you or not." He said: "And what is that, O Umm Hani?" She said: "I was fasting, and I disliked rejecting your favor, so I drank it." He said: "Was it a voluntary fast or an obligatory one?" She said: "I said: Rather, it was voluntary." He said: "The one fasting voluntarily has the choice; if he wishes he may fast, and if he wishes he may break his fast."