Soferim, Hosafah [addition] 1, 1:1: A late midrash relates that Abraham built for Keturah’s offspring a city surrounded by a high iron wall, and brought them inside. He raised the wall so high that sunlight would never enter; and he gave them precious stones and large pearls, which Israel would use in the days to come (Soferim, Hosafah [addition] 1, 1:1).
