Musannaf Ibn Abi Shayba 13:281

Jarir narrated to us, from Mansur, from Mujahid, who said: I was walking with Ibn Umar in the market. We came upon some slave traders who had gathered around a slave girl, turning her over. When they saw Ibn Umar, they moved aside and said, "Ibn Umar has come." Ibn Umar approached her, touched a part of her body, and said, "Where are the owners of this slave girl? She is merely merchandise."