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
Sayyidah Umm Hani (may Allah be pleased with her) narrates: The Messenger of Allah (peace and blessings be upon him) came to me on the day of the Conquest of Makkah and gave me his (peace and blessings be upon him) leftover drink, and I drank it. Then I said: O Messenger of Allah, I was fasting, but I did not like to return your (peace and blessings be upon him) leftover water. He (peace and blessings be upon him) said: "If it was a make-up of Ramadan fasts, fast one day in its place; and if it was a voluntary fast, then it is your choice: make it up if you wish, or do not."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8361
Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates: I prepared food for the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) came to me with his Companions (may Allah be pleased with them). When the food was set down, a man among the people said: I am fasting. The Messenger of Allah (peace and blessings be upon him) said: "Your brother has invited you and gone to trouble for you." Then he said to him: "Break your fast, and fast a day in its place if you wish."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8362