Musnad Ahmad 27:1779

Bahz and Abu Kamil narrated to us, they said: Hammad ibn Salamah narrated to us, Simak ibn Harb narrated to us, from Sa’id ibn Jubayr, from Abdullah ibn Umar, who said: I used to sell camels at al-Baqi’, and I would take silver for gold dinars, and gold dinars for silver. So I came to the Prophet, peace and blessings of Allah be upon him, while he was in the house of Hafsah, and I said: "O Messenger of Allah, wait, I ask you: I used to sell camels at al-Baqi’, and I would take this for that, and that for this." He said: "There is no harm in taking them at the price of their day, as long as you do not separate while there is still something between you."