Abdullah bin Ma'qil (may Allah have mercy on him) said: No land on this side of the mountain is to be sold, except the land of Banu Saluba and the land of al-Hirah, for they have a covenant. Al-Hasan bin Salih (may Allah have mercy on him) said: We used to hear that what lies on this side of the mountain, as against what lies beyond it, was taken by treaty.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18359