Answer:
All praise is due to Allah, exalted is He, and may peace and blessings be upon His Messenger.
Vomiting involuntarily does not break the fast even if it is a mouthful, and even if it goes back down the throat. This is due to the Prophet’s (peace and blessings be upon him) saying: “Three things do not break the fast of a fasting person: vomiting, cupping and having a wet dream”. (Tirmidhi).
However if one deliberates causes the vomiting using fingers or retching etc (al-istiqa ‘amadan) or deliberately swallows it back down (al-i’adah), then the fast will break, but without the need for expiation (kaffarah). The Prophet (peace and blessings be upon him) said, “Whoever is overcome by vomit while he is fasting then making it up (qadha) is not due upon him. But if he vomits intentionally then he should do qadha”. (This hadith is in Abu Dawud and a similar hadith can be found in al-Tirmidhi and al-Muwatta).
According to analogy (qiyas) vomiting, even if initiated by one, should not break the fast just as other things that come out of the body do not break the fast, e.g. urine, stool, etc. However, using analogy in this case has been abandoned due to the hadith that has been mentioned about it.
And Allah knows best.
Answered by: Maulana A. Mushahid Kadir