Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ فِي رِوَايَةِ عَبْدِ اَللَّهِ بْنِ سِنَانٍ قَالَ قَالَ لِي أَبُو عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: «إِذَا كَانَ فِي أَوَّلِ اَلشَّهْرِ خَمِيسَانِ فَصُمْ أَوَّلَهُمَا فَإِنَّهُ أَفْضَلُ وَ إِذَا كَانَ فِي آخِرِ اَلشَّهْرِ خَمِيسَانِ فَصُمْ آخِرَهُمَا فَإِنَّهُ أَفْضَلُ.
In the narration of Abdullah ibn Sinan, he said: "Abu Abdullah (Imam Jafar ibn Muhammad Al-Sadiq عليه السلام) said to me: 'If there are two Thursdays at the beginning of the month, fast the first of them, for it is better. And if there are two Thursdays at the end of the month, fast the last of them, for it is better.'"
- Ṣaḥīḥ al-Faqīh