Yahya related to me from Malik from Salih ibn Kaysan from Hasanibn Muhammad ibn Ali ibn Abi Talib that Ali ibn Abi Talib sold one ofhis camels called Usayfir for 20 camels to be delivered later.
USC-MSA web (English) reference: Book 31, Hadith 59
Arabic reference: Book 31, Hadith 1351