It is reported about a man of the Ansar that he brought a black slave girl and said: O Messenger of Allah! I am obliged to free a believing neck (that is, a male or female slave); if you consider this one to be a believer, then free her. The Prophet (peace and blessings be upon him) asked the slave girl: Do you testify that there is no god but Allah Almighty? She replied: Yes! The Prophet (peace and blessings be upon him) asked: Do you testify that I am the Messenger of Allah? She said: Yes! The Prophet (peace and blessings be upon him) said: Do you believe in being raised again after death? She said: Yes! The Prophet (peace and blessings be upon him) said (to the man): Free her.
Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العتق / 7262