Corollary 5
Tag 09YG, permanent. Cite it as [Bourbaki, Tag 09YG].
Lie Groups and Lie Algebras, chapter I, § 5. Solvable Lie algebras
For a Lie algebra to be solvable, it is necessary and sufficient that be nilpotent.
The condition is necessary by Corollary 1. It is sufficient since is commutative.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.