No prohibition results from suckling, except for what penetrates the intestines from the breast before weaning
Authentic hadithعن أم سلمة قالت: قال رسول الله صلى الله عليه وسلم : «لا يُحَرِّمُ من الرَّضَاعَةِ إلا ما فَتَقَ الْأَمْعَاءَ في الثَّدْيِ، وكان قَبْلَ الْفِطَامِ».
Umm Salamah (may Allah be pleased with her) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: ''No prohibition results from suckling, except for what penetrates the intestines from the breast before weaning.''
Explanation
This Hadīth is evidence that breastfeeding prohibits marriage only when the breast milk reaches and enlarges the intestines. A small amount of milk that does not do this, does not forbid marriage. So, the breastfeeding that affects the permissibility of marriage is what takes place before weaning.
Source · Narrated by At-Termedhy
From the Hadith Encyclopedia (hadeethenc.com).