Umm Hani reported that the Prophet, upon whom be peace, entered her room during the day of the conquest of Makkah. He was offered something to drink and he drank from it. Then he offered it to Umm Hani and she said: “I am fasting.” The Prophet said: “The one who is fasting voluntarily is in charge of himself. If you wish you may fast and if you wish you may break your fast.” This is recounted by Ahmad, ad-Daraqutni, and alBaihaqi. Al-Hakim also related it and said that its chain is sahih. The version he recorded states: “And if one wishes he may fast and if he wishes he may break his fast.”
Abu Juhaifah said: “The Prophet established the bond of brotherhood between Salman and Abu ad-Darda. Once, Salman visited Abu ad-Darda and saw Umm ad-Darda wearing very plain clothes. He said to her: ‘What’s happening to you?’ She said: ‘Your brother Abu ad-Darda has no need in this world.’ When Abu adDarda came, he prepared some food for Salman and said: ‘Eat, for I am fasting.’ Salman said: ‘I shall not eat until you eat.’ So he ate. When it was night, Abu ad-Darda got up to pray and Salman said, ‘Sleep,’ and he did so. Toward the end of the night Salman woke Abu ad-Darda and said, Pray now.’ And they prayed. Salman told him: ‘Your Lord has a right upon you, you have a right upon yourself, and so does your wife. Give each one its due right.’ Abu adDarda went to the Prophet and told him what Salman had said. The Prophet said: ‘Salman has said the truth.'” This is related by al-Bukhari and at-Tirmizhi.
Abu Sa’id al-Khudri said: “I prepared food for the Prophet. He came to me with some of his companions. When the food was laid out, one of the men said: ‘I am fasting.’ The Messenger of Allah said: ‘Your brother has invited you and incurred expenses in your behalf.’ Then he asked [him], Break your fast and fast another day in its place if you wish.'” This is related by al-Baihaqi. Al-Hafizh says it has a hassan chain.
Most scholars maintain that one who is performing a voluntary fast can break it. It is, however, preferred to make up that day later on. The preceding view is clear and authentic hadith are support for that position.