Al-Kāfi - The Book of Fasting

Chapter: The Case of One Who Wakes up in the Morning with an Intention to Fast, but Who Does Not Do So or Vice Versa when Fasting Is a Compensatory Fast for the Missed Fast in the Month of Ramadan or Other Days َّدٍ عَنِ ابْنِ فَضَّالٍ عَنْ صَالِحِ بْنِ عَبْدِ الله الْخَثْعَ٠ِيِّ قَالَ سَأَلْتُ أَبَا عَبْدِ الله (عَلَيْهِ السَّلاَ٠) عَنِ الرَّجُلِ يَنْوِي الصَّوْ٠َ فَيَلْقَاهُ أَخُوهُ الَّذِي هُوَ عَلَى أَ٠ْرِهِ أَ يُفْطِرُ قَالَ إِنْ كَانَ تَطَوُّعاً أَجْزَأَهُ وَحُسِبَ لَهُ وَإِنْ كَانَ قَضَاءَ فَرِيضَةٍ قَضَاهُ.

He (the Imam) said, ‘If it is optional it will be counted as one day of fasting in his favor, but if it is obligatory Qada’ (compensatory fast), he must do its Qada’ (compensatory fast).”’ جهول - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (6/317))