Al-Kāfi

Hadith 138
circumcision
وَ عَنْهُ عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْحُسَيْنِ بْنِ سَعِيدٍ عَنْ فَضَالَةَ بْنِ أَيُّوبَ عَنِ الْقَاسِمِ بْنِ بُرَيْدٍ عَنْ أَبِي بَصِيرٍ عَنْ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) قَالَ مِنْ سُنَنِ الْمُرْسَلِينَ الِاسْتِنْجَاءُ وَ الْخِتَانُ .
6. It is narrated from the narrator of the previous Hadith from Ahmad ibn Muhammad from al-Husayn ibn Sa‘id from Fadalah ibn Ayyub from al-Qasim ibn Burayd from abu Basir who has said the following: “Abu ‘Abd Allah (a.s.), has said, ‘Of the Sunnah of prophets is draining al-Istinja’ (one’s urine) and circumcision.’”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī