Hadith 513420
Chapter 31: Business Transactions - كتاب البيوع
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ سُلَيْمَانَ بْنَ يَسَارٍ، قَالَ فَنِيَ عَلَفُ حِمَارِ سَعْدِ بْنِ أَبِي وَقَّاصٍ فَقَالَ لِغُلاَمِهِ خُذْ مِنْ حِنْطَةِ أَهْلِكَ فَابْتَعْ بِهَا شَعِيرًا وَلاَ تَأْخُذْ إِلاَّ مِثْلَهُ .
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."
Reference Muwatta Malik 513420
USC-MSA web (English) reference : Book 31, Hadith 50