“A man who had intercourse with the slave woman of his wife was brought to Nuʿman 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.' ”  

لاَ أَقْضِي فِيهَا إِلاَّ بِقَضَاءِ رَسُولِ اللَّهِ ﷺ قَالَ إِنْ كَانَتْ أَحَلَّتْهَا لَهُ جَلَدْتُهُ مِائَةً وَإِنْ لَمْ تَكُنْ أَذِنَتْ لَهُ رَجَمْتُهُ  

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.  

أَنَّ رَسُولَ اللَّهِ ﷺ رُفِعَ إِلَيْهِ رَجُلٌ وَطِئَ جَارِيَةَ امْرَأَتِهِ فَلَمْ يَحُدَّهُ