أَخْبَرَنَا عِيسَى بْنُ حَمَّادٍ، قَالَ أَنْبَأَنَا اللَّيْثُ، عَنْ يَزِيدَ بْنِ أَبِي حَبِيبٍ، عَنْ أَبِي الْخَيْرِ، عَنْ عُقْبَةَ بْنِ عَامِرٍ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ إِنَّ أَحَقَّ الشُّرُوطِ أَنْ يُوَفَّى بِهِ مَا اسْتَحْلَلْتُمْ بِهِ الْفُرُوجَ ‏"‏ ‏.‏

It was narrated from 'Utbah bin 'Amir that the Messenger of Allah said: "The conditions that are most deserving of fulfillment, are those by means of which the private parts become allowed to you." Translation Not Available

Book Ref: Sunan An Nisai Book 26 Hadith 3281
Web Ref:  Sunan An Nisai Vol 4 Book 26 Hadith 3283