Parashas Bamidbar · Passuk 2:20
Next to it:The tribe of Manasseh.Chieftain of the Manassites: Gamaliel son of Pedahzur.
Chabad.org — English translation + Rashi
Source: Bamidbar Rabbah 2:10.
English: “And upon him the tribe of Menasheh” — from here [we learn] that “upon him” means adjacent / next to him.
In context: The preposition is not spatial “on top of” but “alongside”; the midrash uses the verse to fix the ordinary sense of the word.
Custom
Chazal derive that “alav” here means in proximity, not literally upon.