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)].

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]‏ .‏