Fasting According To the Five Schools of Islamic Law
The Doubtful Days
There is consensus among the schools that imsak is obligatory upon one who does not fast on a “doubtful day” (yawm al-shakk) that later turns out to be a day of Ramadan, and he is liable to its qada' later.
Where one fasts on a doubtful day that is later known to have been a day of Ramadan, they differ as to whether it suffices without requiring qada':
• The Shafi’i, Maliki and Hanbali schools observe: This fast will not suffice and its qada' is wajib upon him.
• In the opinion of the Hanafis, it suffices and does not require qada'.
• Most Imamis state: Its qada' is not wajib upon him, except when he had fasted with the niyyah of Ramadan.