Jami` at-Tirmidhi 732

Simak bin Harb narrated:"A person from the offspring of Umm Hani narrated to me - I met one of the most virtuous among them, and his name was Ja'dah, and Umm Hani was his grandmother - he narrated to me from his grandmother that the Messenger of Allah entered upon her and asked for some drink, and he drank. Then he offered it to her and she drank it. Then she said: "O Messenger of Allah! I was fasting." So the Messenger of Allah said: "The one fasting a voluntary fast is the trustee for himself; if he wishes he fasts, and if he wishes he breaks." Shu'bah (one of the narrators) said: "I said to him (Ja'dah), 'Did you hear this from Umm Hani?' He said: 'No Abu Salih and our family informed us of it from Umm Hani