From Sahih Muslim (Hadith 3333):
Abu Huraira (Allah be pleased with him) reported that Allah's Messenger (ﷺ) declared sacred the territory between two lava mountains of Medina. Abu Huraira said:If I were to find deer in the territory between the two mountains, I would not molest them, and he (the Holy Prophet) declared twelve miles of suburb around Medina as a prohibited pasture
Reference: Muslim 3333