Hadith: No prohibition results from suckling, except for what penetrates the intestines from the breast before weaning

عن أم سلمة قالت: قال رسول الله صلى الله عليه وسلم : «لا يُحَرِّمُ من الرَّضَاعَةِ إلا ما فَتَقَ الْأَمْعَاءَ في الثَّدْيِ، وكان قَبْلَ الْفِطَامِ».
[صحيح] - [رواه الترمذي]

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.''
Sahih/Authentic. - [At-Tirmidhi]

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.