Proposition 6
Tag 05D2, permanent. Cite it as [Bourbaki, Tag 05D2].
Algebra, chapter I, § 6. Extensions, solvable groups, nilpotent groups
Let be a group homomorphism. Then . If is surjective, the homomorphism of into the restriction of is surjective.
The image under of a commutator of elements of is a commutator of elements of . If is surjective, the image under of the set of commutators of is the set of commutators of . The proposition thus follows from § 4, no. 3, Corollary 3 to Proposition 2.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.