"The Messenger of Allah swore to keep away from his wives and declared them as unlawful for him, so he made something permissible forbidden, and he offered expiation for having sworn to do so."  

آلَى رَسُولُ اللَّهِ ﷺ مِنْ نِسَائِهِ وَحَرَّمَ فَجَعَلَ الْحَرَامَ حَلاَلاً وَجَعَلَ فِي الْيَمِينِ كَفَّارَةً  

"For the one who makes unlawful is the swearing." (Sahih)And Ibn 'Abbas used to say: "You had the best example in the Messenger of Allah." (33:21)  

ابْنُ عَبَّاسٍ فِي الْحَرَامِ يَمِينٌ وَكَانَ ابْنُ عَبَّاسٍ يَقُولُ لَقَدْ كَانَ لَكُمْ فِي رَسُولِ اللَّهِ أُسْوَةٌ حَسَنَةٌ