Riyad as-Salihin 17:93

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] .