birds of a feather (flock together)

Individuals of like character, taste, or background (tend to stay together), as in The members of the club had no trouble selecting their yearly outing—they're all birds of a feather. The idea of like seeks like dates from ancient Greek times, and “Birds dwell with their kind” was quoted in the apocryphal book of Ecclesiasticus. The full saying in English, Birds of a feather flock together, was first recorded in 1545.