Proposition 3
Tag 0A6W, permanent. Cite it as [Bourbaki, Tag 0A6W].
Lie Groups and Lie Algebras, chapter II, § 8. Convergence of the Hausdorff series (ultrametric case)
Let be the ball . The analytic mapping
makes into a group in which is the identity element and is the inverse of for all . Moreover, if is a real number such that , the ball
(resp. ) is an open subgroup of .
As and , and
for all . It therefore remains to prove the associativity formula
As
in (§ 6, no. 5, Proposition 4), we have
in (no. 2) and (19) implies (18) by (16) and Differentiable and Analytic Manifolds, R, 4.1.5.
In other words (Chapter III, § 1), with the Hausdorff function is a Lie group.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.