What is the evidence that it is permissible to withdraw from a voluntary fast?

The Shari'ah encourages voluntary fasting to increase reward and draw closer to Allah. If a person begins a voluntary fast, they are not obligated to complete it, nor to make up for it if they break their fast. This is based on the narration of Aisha (may Allah be pleased with her): "Some hais (a dish of dates, ghee, and cheese) was brought to us, and the Prophet (peace and blessings of Allah be upon him) said: 'Bring it near, for I have become fasting,' and he ate." Narrated by Muslim. An-Nasa'i added: "The example of a volunteer is like a man who gives charity from his wealth; if he wishes, he completes it, and if he wishes, he withholds it." And based on the saying of the Prophet (peace and blessings of Allah be upon him): "The one who is fasting voluntarily is his own master; if he wishes, he fasts, and if he wishes, he breaks his fast." Narrated by Ahmad and At-Tirmidhi, and authenticated by Al-Albani. However, it is recommended to complete a supererogatory fast, based on the saying of Allah the Almighty: (And do not invalidate your deeds).