Musannaf Ibn Abi Shayba 13:2468

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from Muslim, he said: I asked Muhammad ibn Sirin about a man who sold a commodity for two months, and he stipulated to the buyer that if he sold it before the two months, he must pay him in cash. He said: I do not see any harm in it.