Not graded
Bulugh al-Maram 1080b
وَلِمُسْلِمٍ : { إِذَا حَرَّمَ اَلرَّجُلُ عَلَيْهِ اِمْرَأَتَهُ , فَهِيَ يَمِينٌ يُكَفِّرُهَا } 1 .
1 - صحيح . رواه مسلم ( 1473 ).
Muslim has:
"When a man makes his wife unlawful for himself, it is (treated like) an oath for which atonement must be made (if broken)."
Cited in Bulugh al-Maram (No. 1080b).