The World Federation One Stop Fiqh

Ruling 1590

Whenever someone who wants to keep a qaḍāʾ fast of the month of Ramadan intentionally remains in the state of janābah until the time of ṣubḥ prayers, he cannot fast on that day. If he does this unintentionally, he can fast on that day, although the recommended precaution is that he should not [fast on that day, and should instead fast on another day].