Yahya bin Abi Kathir يحيى بن أبي كثير الطائي · The Book on Business

"I asked Messenger of Allah (ﷺ), I said: 'A man came to me asking to buy something that I did not have. Can I buy it from the market for him and then give it to him?' He said: 'Do not sell what is not with you.'" He said: There are narrations on this topic from 'Abdullah bin 'Umar.

"During the year of the Farewell Pilgrimage, I heard the Prophet (ﷺ) saying during the Khutbah: "The borrowed is to be returned, and the guarantor is responsible, and the debt is to be repaid." There are narrations on this topic from Samurah, Safwan bin Umayyah, and Anas. The Hadith of Abu Umamah is a Hasan Gharib Hadith. It has also been reported through other routes besides this, from Abu Umamah, from the Prophet (ﷺ).