Al-Kāfi - The Book of Fasting

Chapter: The Case of One Who Vows to Fast a Certain Day or for Thanksgiving ِنْ أَصْحَابِنَا عَنْ أَحْ٠َّدٍ عَنْ عَلِيِّ بْنِ أَحْ٠َ قَالَ كَتَبَ الْحُسَيْنُ إِلَى الرِّضَا (عَلَيْهِ السَّلاَ٠) جُعِلْتُ فِدَاكَ رَجُلٌ نَذَرَ أَنْ يَصُو٠َّ اعْتَلَّ فَأَفْطَرَ أَ يَبْتَدِئُ فِي صَوْ٠َضَى فَكَتَبَ إِلَيْهِ يَحْتَسِبُ ٠2.

A number of our people have narrated from Ahmad ibn Muhammad from Ali ibn Ahmad ibn ‘Ushaym who has said the following: “Al-Husayn once wrote to al-Rida’ (a.s.), saying, ‘I pray to Allah to keep my soul in service for your cause, a man had vowed to fast for certain days and fasted a few days, then he became ill and stopped fasting.

Is he required to start fasting all over again or are the days that he had fasted are counted?’ He (the Imam) wrote the answer that said, ‘The days that he had fasted are also counted.’” جهول - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (6/350))