Sa'id bin Jubayr (may Allah have mercy on him) narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): I used to sell camels at al-Baqi', and dirhams would accumulate with me. I would sell them to a man for dinars, and he would give them to me the next morning. I spoke to the Messenger of Allah (peace and blessings be upon him) about this, and he (peace and blessings be upon him) said: "When you trade with a man in gold and silver, do not part from him while there is some matter or ambiguity (outstanding) between you."
Among the students of Ibn Umar, Simak bin Harb, from Sa'id bin Jubayr, is alone in narrating this hadith as marfu' (from the Prophet).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10514