On the authority of Anas ibn Malik, that the Prophet, peace and blessings be upon him, said: "A townsman shall not sell for a Bedouin, even if he is his brother or his father."(1)

Abu Dawud said: I heard Hafs ibn Umar say: Abu Hilal narrated to us, Muhammad narrated to us, on the authority of Anas ibn Malik, who said: It used to be said: A townsman shall not sell for a Bedouin, and it is a comprehensive statement—he neither sells anything for him, nor buys anything for him.

3441 - Musa ibn Isma'il narrated to us, Hammad narrated to us, on the authority of Muhammad ibn Ishaq, on the authority of Salim al-Makki, that a Bedouin told him that he came with his merchandise for sale during the time of the Messenger of Allah, peace and blessings be upon him, and he lodged with Talhah ibn Ubayd Allah, who said: The Prophet, peace and blessings be upon him, forbade that a townsman sell for a Bedouin. But go to the market and see who will trade with you, and consult me so that I may command you or forbid you.(2)

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(1) A sound hadith, and this chain's narrators are trustworthy, and al-Hasan—namely al-Basri—although he did not explicitly state that he heard from Anas, is corroborated. Yunus: he is ibn Ubayd.
It was also recorded by al-Nasa'i (4492) via the chain of Muhammad ibn al-Zibriqan, with this chain.
It was also recorded by al-Bukhari (2161), Muslim (1523), and al-Nasa'i (4493) and (4494) via the chain of Muhammad ibn Sirin, on the authority of Anas ibn Malik, with the wording: "We were forbidden that a townsman sell for a Bedouin."
And his statement in the hadith, "A townsman shall not sell," is a negation with the meaning of prohibition.

(2) A good hadith, and in this chain Hammad—namely ibn Salamah—erred in attributing Salim, the teacher of Ibn Ishaq, as al-Makki, whereas he is Salim ibn Abi Umayyah Abu al-Nadr al-Madani, the trustworthy. It came correctly in the narration of Ibrahim ibn Sa'd and Yazid ibn Zuray' from Ibn Ishaq, and in their narration Ibn Ishaq explicitly stated that he heard from Salim Abu al-Nadr, and it was clarified therein that this Bedouin came to the Prophet, peace and blessings be upon him, he and his father, and that he wrote for them a document: that no one should transgress against them regarding their alms, so the chain through their two routes is good.
It was also recorded by al-Bazzar in his Musnad (957), and Abu Ya'la (643), via two routes from Hammad ibn Salamah, with it. =