Hadith #2288 (Sunan Ibn Majah)
Book: The Chapters on Legal Punishments
Narrator(s): Abdullah bin 'Uthman bin Khuthaym عبد الله بن عثمان بن خثيم القاري, Qayla, Umm Bani Anmar, Y'aqub bin Hmyd bin Kasb يعقوب بن حميد بن كاسب المدني

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.

Source: https://parallelquran.com/hadith/sunan-ibn-majah-2288/