Musnad Ahmad 462:31

Abu al-Yaman narrated to us, he said: Abu Bakr ibn Abi Maryam narrated to us, he said: Miqdam ibn Ma'di Karib had a slave girl who used to sell milk, and Miqdam would collect the price. It was said to him: "Glory be to Allah! Do you sell milk and collect the price?" He said: "Yes, and what is wrong with that? I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'A time will surely come upon the people in which nothing will be of benefit except the dinar and the dirham.'"