Sahih Muslim 3549
صحيح مسلم#3549SahihAbu Sa'id al-Khudri (Allah be pleased with him) reported that Allah's Apostle (ﷺ) was asked about 'azl, whereupon he said:There is no harm if you do not do that, for it (the birth of the child) is something ordained. Muhammad (one of the narrators) said: (The words) La 'alaykum (there is no harm) implies its Prohibition
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 16:151