Sunan an-Nasai 19239
In-book no 4503أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنِي سَالِمُ بْنُ نُوحٍ، قَالَ أَنْبَأَنَا يُونُسُ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ نُهِينَا أَنْ يَبِيعَ، حَاضِرٌ لِبَادٍ وَإِنْ كَانَ أَخَاهُ أَوْ أَبَاهُ .
It was narrated that Anas bin Malik said: "It was forbidden to us for a town-dweller to sell for a desert-dweller, even if he was his father or brother." (Sahih )