Theorem 1
Tag 0AE9, permanent. Cite it as [Bourbaki, Tag 0AE9].
Lie Groups and Lie Algebras, chapter III, § 6. Real and complex Lie groups
Let and be Lie groups and a continuous morphism of into . Suppose that is simply connected. Then there exists one and only one Lie group morphism of into such that .
The existence of follows from Lemma 1 and § 4, no. 1, Theorem 1 (i). The uniqueness of follows from § 4, no. 1, Theorem 1 (ii) and the fact that is connected.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.