Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ رَوَى اَلْحَسَنُ بْنُ مَحْبُوبٍ عَنِ اَلْحَسَنِ بْنِ أَبِي حَمْزَةَ قَالَ: قُلْتُ لِأَبِي جَعْفَرٍ أَوْ لِأَبِي عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ صَوْمُ ثَلاَثَةِ أَيَّامٍ فِي اَلشَّهْرِ أُؤَخِّرُهُ فِي اَلصَّيْفِ إِلَى اَلشِّتَاءِ فَإِنِّي أَجِدُهُ أَهْوَنَ عَلَيَّ فَقَالَ «نَعَمْ فَاحْفَظْهَا ».
It is narrated by al-Hasan ibn Mahbub from al-Hasan ibn Abi Hamzah, who said: "I said to Abu Ja‘far or Abu Abdullah عليه السلام: 'Can I delay fasting the three days of the month in the summer until the winter, as I find it easier for me then?' Imam عليه السلام said: 'Yes, but make sure to observe it.'"
- Ṣaḥīḥ al-Faqīh