Yes, it is narrated in Sahih Muslim multiple times.
عَائِشَةَ أَخْبَرَتْهُ أَنَّ سَهْلَةَ بِنْتَ سُهَيْلِ بْنِ عَمْرٍو جَاءَتِ النَّبِيَّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فَقَالَتْ: يَا رَسُولَ اللهِ إِنَّ سَالِمًا – لِسَالِمٍ مَوْلَى أَبِي حُذَيْفَةَ – مَعَنَا فِي بَيْتِنَا. وَقَدْ بَلَغَ مَا يَبْلُغُ الرِّجَالُ، وَعَلِمَ مَا يَعْلَمُ الرِّجَالُ. قَالَ: «أَرْضِعِيهِ تَحْرُمِي عَلَيْهِ»
(Sayidah) Ayesha narrated that Sahlah bint Suhayl bin Amr came to the Prophet ﷺ and said, O Messenger of Allah ﷺ indeed Salim (Salim was the freed slave of Abu Hudhaifah) is with us in our house, and he has become a man and knows what men know. He ﷺ said, ‘Give him breastmilk, he will become Mahram to you.’ (Sahih Muslim, 1453 – 28).
This is one narration of this Hadith in Sahih Muslim, there are many other too see 1453 – 27, 1453 – 29, 1453 – 30, 1695 – 23. Musnad Hadith that have been narrated in Bukhari or Muslim are considered to be Sahih.
However, this Hadith is considered to be Khass, specific to that situation, and not a general ruling. See the links to answer below for further details.
And Allah (Most High) Knows Best.
– Answered by Shaykh Noorud-deen (23.03.2021)
Explaining the Hadith of giving breastmilk to Sayidina Salim?
All Musnad Hadith in Bukhari and Muslim are Sahih
See also (video):