قُلْتُ لَهُ رَجُلٌ أَسَرَتْهُ الرُّومُ وَ لَمْ يَصُمْ شَهْرَ رَمَضَانَ وَ لَمْ يَدْرِ أَيُّ شَهْرٍ هُوَ قَالَ يَصُومُ شَهْراً وَ يَتَوَخَّاهُ وَ يَحْسُبُ فَإِنْ كَانَ الشَّهْرُ الَّذِي صَامَهُ قَبْلَ شَهْرِ رَمَضَانَ لَمْ يُجْزِهِ وَ إِنْ كَانَ بَعْدَ رَمَضَانَ أَجْزَأَهُ.
“I once asked abu ‘Abd Allah (a.s.), about a man who was captured by the Romans, and he did not fast in the month of Ramadan because he did not know which month was it. He (the Imam) said, ‘He is required to fast for one month. He then tries to calculate; if the month that he has fasted is before the month of Ramadan;