Musnad Ahmad 27:1967

Yahya ibn Adam narrated to us, Israel narrated to us, from Simak, from Sa'id ibn Jubayr, from Ibn Umar, who said: I used to sell camels at al-Baqi', so I would sell for dinars and take dirhams, and I would sell for dirhams and take dinars. So I came to the Prophet, peace and blessings of Allah be upon him, while he was intending to enter his chamber, and I took hold of his garment and asked him. He said: "If you take one of the two for the other, then do not let him depart from you while there is still a transaction between you and him."