He traced it back to the Prophet ﷺ, who said, "A town dweller must not sell on behalf of a desert dweller."
يبلغ به النبي ﷺ قال: لا يبع حاضر لباد.
Bukhari related it in the Book of Sales (2140), and Muslim in the Book of Sales (1520), both of them through Sufyan, from Zuhri, from Saeed bin Musayab, from Abu Hurairah, who narrated it. The wording given is that of Muslim, while Bukhari presented it in a fuller form, which he mentioned in its proper place.