Abu Ali Suwaid bin Muqarrin (May Allah be pleased with him) said:
I was the seventh child of Banu Muqarrin and we had only one slave-girl. When the youngest of us once happened to slap her (on the face) the Messenger of Allah (ﷺ) ordered us to set her free.[Muslim].
App reference: Book 18, Hadith 93
Arabic/English book reference: Book 18, Hadith 1603