It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that I heard the Prophet (peace and blessings be upon him) say: When you exchange gold for silver or either of these things, do not separate from your companion as long as there is even the slightest doubt between you and him.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5237