Umm Hani bint Abi Talib (may Allah be pleased with her) narrates: The Messenger of Allah (peace and blessings be upon him) came to me. I called for water for him (peace and blessings be upon him) and he drank it, then he gave it to me and I drank too, and I said: O Messenger of Allah, I was fasting, but I did not want to return your (peace and blessings be upon him) leftover water. The Messenger of Allah (peace and blessings be upon him) said: "If it was a make-up fast for Ramadan, then fast a day in its place; and if it was a voluntary fast, then if you wish, make it up, and if you wish, do not make it up."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8360