Musannaf Ibn Abi Shayba 13:79

Ibn Fudayl narrated to us, from Bukayr ibn Atiq, who said: I said to Sa’id ibn Jubayr, "Should I take collateral in a *salam* (forward sale) contract?" He said, "That is a guaranteed profit." He said: I said, "Should I take a guarantor?" He said, "That is a guaranteed profit."