the case of a man who had intercourse with the slave woman of his wife was referred to the Messenger of Allah (ﷺ), and he did not stipulate any legal punishment for him.
Sunan Ibn Majah Book 20, Hadith 20
← Previous Hadith “A man who had intercourse with the slave woman of his wife was brought to Nu`man bin Bashir. He said: 'I will pass no other judgement than that of the Messenger of Allah (ﷺ) He said: 'If (his wife) had made her lawful for him, then I will give him one hundred lashes, but if she has not given permission, I will stone him.' ”

Sunan Ibn Majah Book 20, Hadith 19 Next Hadith → “I fear that after a long time has passed, some will say: 'I do not find (the sentence of) stoning in the Book of Allah (ﷺ),' and they will go astray by abandoning one of the obligations enjoined by Allah (SWT). Rather stoning is a must if a man is married (or previously married) and proof is established, or if pregnancy results or if he admits it. I have read it (in the Quran). “And if an old man and an old woman commit adultery, stone them both.” The Messenger of Allah (ﷺ) stoned (adulterers) and we stoned (them) after him.' ”

Sunan Ibn Majah Book 20, Hadith 21