Let ρ:A→B be a homomorphism of noetherian rings. Then
dim(B)≤dim(A)+p∈Spec(A)supdim(B⊗Aκ(p)).
Let q be a prime ideal of B and p=ρ−1(q). By Cor. 1, we have dim(Bq)≤dim(Ap)+dim(Bq⊗Aκ(p)). It follows (Prop. 6, b) of § 1, No. 3) that we have the inequality dim(Bq)≤dim(A)+dim(B⊗Aκ(p)), and one concludes by Prop. 8 of § 1, No. 3.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.