Answered by Shaykh Amjad Rasheed
If dawn has come in, and I want to fast, but there is food in my mouth at the time, do I swallow it or spit it out?
If you intended to fast an obligatory fast, then it is obligatory for you to take it out, for if you swallowed it, then your fast is invalid and it is obligatory on you to make it up, and you have committed a sin by doing that. There is no difference of opinion in this, as Imam al-Nawawi said in the Majmu` (6:311). The scholars have consensus (ijma`) that the beginning of the fast is dawn, and Allah, Blessed and Most High, has narrated that the limit to eating and drinking is dawn, for He has said, And eat and drink until the white thread of dawn appear to you distinct from its black thread. [Quran]
The same is said about a supererogatory fast, if one swallows food after Fajr enters this invalidates the fast. However, one has not committed a sin, because according to our school, breaking a supererogatory fast is permissible, but disliked.
And as for what was related by our master Abu Hurayra, may Allah be well pleased with him, on the authority of the Prophet, peace and blessings be upon him, that he said, When one of you has heard the call, and the container is still in his hand, do not put it down until you have fulfilled your need., this is a weak hadith, as the imams of hadith have said, such as Imam Abu Hatim al-Razi and others.
[Translated by Sr. Shazia Ahmad]