In his well-known book, Fiqh As-Sunnah, the late Sheikh Sayyed Sabiq states: “Umm Hani reported that the Prophet (peace and blessings be upon him) 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 (peace and blessings be upon him) 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.” (Reported by Ahmad)
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 ad-Darda’ 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 your wife has a right upon you. Give each one its due right.’ Abu ad-Darda’ went to the Prophet and told him what Salman had said. The Prophet said: ‘Salman has said the truth.’” (Reported by al-Bukhari)
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 served, 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.’” (Reported by al-Bayhaqi)
Most scholars maintain that one who is performing a voluntary fast can break it. It is, however, preferred to make up for that day later on. The preceding view is clear and supported by an authentic hadith.”