Abu Salih narrates from Umm Hani (may Allah be pleased with her): The Messenger of Allah (peace and blessings be upon him) came to me; he (peace and blessings be upon him) asked for water and drank it, and gave me what was left over. I was fasting, but I drank it. I said: O Messenger of Allah, I have done something and I do not know whether I did right or wrong. You (peace and blessings be upon him) gave me your leftover water while I was fasting, and I drank it because I disliked returning your leftover (peace and blessings be upon him). He (peace and blessings be upon him) said: "Were you observing a voluntary fast or making up a fast of Ramadan?" I said: Voluntary. He (peace and blessings be upon him) said: "A voluntary fast is by choice: if you wish, keep it, and if you wish, break it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8346