The World Federation One Stop Fiqh

Ruling 1545

If a disbeliever (kāfir) becomes a Muslim during the daytime in the month of Ramadan, and from the time of ṣubḥ prayers until the time he became a Muslim he did not do anything that invalidates a fast, then based on obligatory precaution, he must abstain [from the eight things that invalidate a fast] until the end of the day with the intention to fulfil whatever his legal obligation happens to be (mā fī al‐dhimmah).If he does not do this, he must keep a qaḍāʾ fast for it.