Maliks Muwatta Book 31, Hadith Number 51.

Section : Bartering Food for Food with No Increase between Them.

Yahya related to me from Malik from Nafi that Sulayman ibn Yasar told him that one time the fodder of the animals of Abd ar-Rahman ibn al-Aswad ibn Abd Yaghuth was finished so he said to his slave, “Take some of your family’s wheat as food and buy with it barley, and take only a like quantity.'”