Chapter on the Days of Sacrifice
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 3|Chapter 134
وَ رُوِي: «أَنَّ اَلْأَضْحَى ثَلاَثَةُ أَيَّامٍ وَ أَفْضَلُهَا أَوَّلُهَا ».
It is narrated: "The Eid al-Adha (sacrifice) lasts for three days, and the first day is the most virtuous."
- Ṣaḥīḥ al-Faqīh