Hadith: Suckle him, and you will become unlawful for him (in marriage), and (the discomfort) which Abu Hudhayfah feels in his heart will disappear

عن عائشة رضي الله عنها أن سالما مولى أبي حذيفة كان مع أبي حذيفة وأهله في بيتهم، فأتت -تعني ابنة سُهيل- النبي صلى الله عليه وسلم فقالت: إن سالما قد بلغ ما يبلغ الرجال. وعَقَل ما عقلوا. وإنه يدخل علينا. وإني أظن أن في نفس أبي حذيفة من ذلك شيئا. فقال لها النبي صلى الله عليه وسلم «أَرْضِعِيهِ، تَحْرُمِي عليه، ويذهب الذي في نفس أبي حذيفة» فرجعت فقالت: إني قد أرضعته. فذهب الذي في نفس أبي حذيفة.
[صحيح] - [رواه مسلم]

‘Ā’ishah (may Allah be pleased with her) reported that Sālim, the freed slave of Abu Hudhayfah, was with Abu Hudhayfah and his family in their home. Bint Suhayl came to the Prophet (may Allah's peace and blessings be upon him) and said: "Sālim has attained puberty, and he now perceives what they perceive, yet he enters our house freely, and I believe that something (troubles) the heart of Abu Hudhayfah." Thereupon the Prophet (may Allah's peace and blessings be upon him) said to her: "Suckle him, and you will become unlawful for him (in marriage), and (the discomfort) which Abu Hudhayfah feels in his heart will disappear." She returned and said: "So I suckled him, and what (was there) in the heart of Abu Hudhayfah disappeared."
Sahih/Authentic. - [Muslim]

Explanation

Sahlah bint Suhayl, the wife of Abu Hudhayfah, sought a legal opinion concerning Sālim, one of the meritorious Companions. Her husband Abu Hudhayfah had adopted Sālim when adoption was still permissible and was not yet abrogated. Sālim grew up under the care of Abu Hudhayfah and his wife like a son to both of them. However, when Allah, the Almighty, revealed: {Call them by [the names of] their fathers}, adoption became forbidden. As long as he was still young, Sālim was entering and exiting the house in which Sahlah lived. He accessed the house freely until he grew up into an adult man, and he would still see Sahlah. Abu Hudhayfah did not like that, but it was difficult for the spouses to deny Sālim access given their amicable relationship with him. Hence, they asked the Messenger of Allah (may Allah's peace and blessings be upon him) about that, to which he said to Sahlah: "Suckle him so that you will be forbidden to him." Thus Abu Hudhayfah's aversion would wear out. Sahlah suckled him, and the matter was settled. It should be noted, however, that this ruling is specific to that case, for anyone who was suckled by a woman after he had been weaned, the sucking woman would not become his foster mother, as the Permanent Committee stated.