It was narrated from 'Utbah bin 'Abd Sulamain:
that the Messenger of Allah said: "When anyone of you has intercourse with his wife, let him cover himself and not be naked liked donkeys." (ID)
English reference (ID): Vol. 3, Book 9, Hadith 1921 (ID)
Arabic reference (ID): Book 9, Hadith 1996 (ID)