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; but 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 instead fast on another day].