Hadith: Breastfeeding produces the same interdiction which childbirth produces

عن عائشة رضي الله عنها عن النبي صلى الله عليه وسلم قال: «الرَّضاعة تحرم ما تحرم الولادة».
[صحيح] - [متفق عليه]

‘Ā'ishah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: ''Breastfeeding produces the same interdiction which childbirth produces."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The women whom a man is prohibited from marrying because of his blood relations with them, such as the mother and sister, are also prohibited to him through breastfeeding, such as the suckling mother (wet nurse) and suckling sister (who suckled from the same woman as him). Therefore, another Hadīth reads: "Breastfeeding forbids (for marriage) what lineage (blood relations) forbids." This is the case whether it's on the wife's side or on the husband's side. All women whom a man is forbidden to marry because of kinship – like his sister, maternal aunt, and paternal aunt – are also forbidden for him to marry if he is related to them by breastfeeding. Likewise is the wife; just as she is forbidden to marry her son, brother, paternal uncle, and maternal uncle, she is also forbidden to marry them if they are related to her only through breastfeeding. Breastfeeding forbids marriage and establishes a Mahram status between the suckled infant and the opposite-sex children of the suckling mother. They are considered as relatives with regard to the lawfulness of looking freely at them (without having to lower his gaze), accompanying them in travel, and being alone with them in a place. However, the breastfeeding relation has no effect on other rulings, such as inheritance, the obligation to provide for each other, and the like. Also, the mentioned prohibition applies to the suckling mother, as her relatives are relatives of the suckled child, but the relatives of the suckled child, apart from his offspring, are not related to the suckling mother. None of the rulings applies to them.