Corollary 1
Tag 00TS, permanent. Cite it as [Bourbaki, Tag 00TS].
Lie Groups and Lie Algebras, chapter VII, § 2. Cartan subalgebras and regular elements of a Lie algebra
Let be a Lie algebra, a Cartan subalgebra of . If is infinite, there exists such that .
Indeed, and we can apply Prop. 9 (ii) of §1.
Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.