Silsilat al-Ahadith al-Sahihah

Narrated from Sayyidah Umm Hani (may Allah be pleased with her), she says: The Messenger of Allah (peace and blessings be upon him) drank water and handed me the remaining water so that I could drink it. I said: I was fasting, but I also disliked returning what was left by you. The Messenger of Allah (peace and blessings be upon him) said: "If this fast is a make-up for Ramadan, then make it up, and if it is voluntary, then (it is up to you) if you wish, make it up, and if you wish, do not make it up."