Let X and X′ be complete normable spaces, X0 an open neighbourhood of 0 in X and f an analytic mapping of X0 into X′ such that f(0)=0. Let f=f1+f2+f3+⋯ be the expansion of f as an integral series about 0, where fi is a homogeneous continuous-polynomial of degree i on X with values in X′. Let t be an element of TS2(X), considered as a point distribution on X with support contained in \{0\}. Let t′=f∗(t)∈TS(X′). The homogeneous component of t′ of degree 1 is ⟨f2,t⟩.
Let t1′ be this component. Then, for every continuous linear mapping u of X′ into a polynormed space,
u(t1′)=⟨t′,u⟩because u is continuous and linear
=⟨t,u∘f⟩(Diff. & Anal. Man., R, 13.2.3)
=⟨t,u∘f2⟩because t∈TS2(X)
=u(⟨t,f2⟩)(Diff. & Anal. Man., R, 13.2.2),
whence the lemma.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.