Musannaf Ibn Abi Shayba 13:2259

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Mu’tamir ibn Sulayman, from Salam ibn Abi al-Dhayyal, he said: I asked Muhammad 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 immediately. He said: I do not see any harm in it.