Abu Dawud Book 27, Hadith Number 4102.

Chapter : On the meaning of the quranic verse and say to the beleiving woman that they should lower their gaze.

Narated By Abdullah ibn Amr ibn al-‘As : The Prophet (PBUH) said: When one of you marries his female servant to his slave or to his employee, he should not look at her private part below the navel and above the knees.