Sunan an-Nasa'i, Book 26, Hadith 86 (In-Book) ⓘ
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."
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 " .
also referenced under: Hadith 3281 (Reference) & Volume 4, Book 26, Hadith 3283 (DarusSalam)