It was narrated from 'Uqbah bin 'Amir:
that the Prophet said: "The conditions most deserving to be fulfilled are those by means of which the private parts become permissible for you." (RU)
English reference (RU): Vol. 3, Book 9, Hadith 1954 (RU)
Arabic reference (RU): Book 9, Hadith 2030 (RU)