Chapter on Breastfeeding
Man Lā Yaḥḍuruh al-Faqīh|Volume 3|Book 3|Chapter 47
وَ رُوِيَ عَنْ أَيُّوبَ بْنِ نُوحٍ قَالَ كَتَبَ عَلِيُّ بْنُ شُعَيْبٍ إِلَى أَبِي اَلْحَسَنِ عَلَيْهِ اَلسَّلاَمُ: اِمْرَأَةٌ أَرْضَعَتْ بَعْضَ وُلْدِي هَلْ يَجُوزُ لِي أَنْ أَتَزَوَّجَ بَعْضَ وُلْدِهَا فَكَتَبَ "لاَ يَجُوزُ ذَلِكَ لِأَنَّ وُلْدَهَا قَدْ صَارَ بِمَنْزِلَةِ وُلْدِكَ".
It was narrated from Ayyub ibn Nuh who said: Ali ibn Shu'ayb wrote to Abu al-Hasan عليه السلام asking: "A woman breastfed one of my children. Is it permissible for me to marry one of her children?" Imam عليه السلام wrote in response, "That is not permissible because her children have become like your own children."
- Ṣaḥīḥ al-Faqīh, p. 291