Numbers 35:4-5

The open land around the cities you give to the L'vi'im is to commence at a line drawn around the city wall 1,500 feet outside it and is to extend outward from there.

Measure 3,000 feet outward from the city wall to the east, south, west and north, with the city in the center. The space between the 1,500-foot line and the 3,000-foot line will be their open land around the cities.