بُلُوغُ الْمَرَامِ
Bulugh al-Maram 1133
Marriage
وَعَنْ أُمِّ سَلَمَةَ رَضِيَ اَللَّهُ عَنْهَا قَالَتْ: قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ لَا يُحَرِّمُ مِنْ اَلرَّضَاعَةِ إِلَّا مَا فَتَقَ اَلْأَمْعَاءَ, وَكَانَ قَبْلَ اَلْفِطَامِ. } رَوَاهُ اَلتِّرْمِذِيُّ, وَصَحَّحَهُ هُوَ وَالْحَاكِمُ [1] .
Narrated Umm Salamah (RA): Allah's Messenger (ﷺ) said: "The suckling that makes marriage unlawful is that which reaches the intestines (i.e. nourishes the child), and is taken before (two years) time of weaning." [Reported by at-Tirmidhi, he and al-Hakim graded it Sahih (authentic)].
Sunnah.com reference: Book 8, Hadith 197