Hadith.1920 - Aban ibn Uthman narrated from Abd al-Rahman ibn Abi Abdullah, from Abu Abdullah عليه السلام: I asked him about a man who was captured by the Romans and did not know when the Month of Ramadan occurred. Imam عليه السلام said: "He should fast for a month, making an effort to estimate and calculate. If the month he fasted was before the Month of Ramadan, it does not suffice. However, if it was after the Month of Ramadan, it suffices."