Al-Kāfi - Volume 4 > Postponing the Three Days Fasting to Winter Months
Hadith #1

1ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنِ الْحَسَنِ بْنِ رَاشِدٍ قَالَ قُلْتُ لأَبِي عَبْدِ الله أَوْ لأَبِي الْحَسَنِ (عَلَيْهِ السَّلاَم) الرَّجُلُ يَتَعَمَّدُ الشَّهْرَ فِي الأَيَّامِ الْقِصَارِ يَصُومُهُ لِسَنَةٍ قَالَ لا بَأْسَ.

1. Ali ibn Ibrahim has narrated from his father from ibn abu ‘Umayr from al-Hassan ibn Rashid who has said the following: “I once asked abu ‘ Abd Allah, ‘Alayhi al-Salam, about a man who purposely postpones fasting for the times when days of the year are shorter. He (the Imam) said, ‘It is not harmful.’”