Musannaf Ibn Abi Shayba 13:2085

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Abu Ka'b 'Abd Rabbih ibn 'Ubayd narrated to us, he said: I asked Ibn Sirin about the selling of silk. He said: A man would buy the merchandise, then he would set it aside. If he found a profit in cash, he would sell it, and if he found a profit on credit, he would sell it.