Abu Dawud Book 15, Hadith Number 3277.

Narated By Ash-Sharid ibn Suwayd ath-Thaqafi : Sharid’s mother left a will to emancipate a believing slave on her behalf. So he came to the Prophet (PBUH) and said: Apostle of Allah, my mother left a will that I should emancipate a believing slave for her, and I have a black Nubian slave-girl. He mentioned a tradition about the test of the girl.