Corollary 2
Tag 0AEO, permanent. Cite it as [Bourbaki, Tag 0AEO].
Lie Groups and Lie Algebras, chapter III, § 6. Real and complex Lie groups
Let G be a Lie group and H_1 and H_2 integral subgroups of G. Suppose that the topology on H_1 admits a countable base. Then
and, if these conditions hold, H_2 is an integral subgroup of H_1.
The last assertion and the implication follow from Corollary 2 to Proposition 1. The converse implication follows from Proposition 3.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.