Maliks Muwatta Book 31, Hadith Number 50.
Section : Bartering Food for Food with No Increase between Them.
Yahya related to me from Malik that he had heard that Sulayman ibn Yasar said, “The fodder of the donkeys of Saad ibn Abi Waqqas ran out and so he told his slave to take some of the family’s wheat and buy barley with it, and to only take a like quantity.”