Anas bin Malik (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) forbade that a person in the city should make a deal on behalf of a villager on commission even if he is his real brother."[Al-Bukhari and Muslim] (ES)
App reference (ES): Book 18, Hadith 265 (ES)
Arabic/English book reference (ES): Book 18, Hadith 1775 (ES)