Let $G$ be some (dimension $1$, to simplify) formal group over a characteristic $0$ field $K$. The law of $G$ is denoted by $\oplus$. If $w(X) \in K[[X]] dX$ is a differential form, let $F_w(X)$ be the unique power series such that $dF_w=w$ and $F_w(0)=0$. Let $F_w^2(X,Y) = F_w(X \oplus Y) - F_w(X) - F_w(Y)$. Say that $w$ is second kind if $F_w^2$ has bounded coefficients and that $F_w$ is exact if $F_w$ has bounded coefficients. The 1st de Rham cohomology group of $G$ is defined by $$H^1_{dR}(G)= \text{\{second kind forms\}} / \text{\{exact forms\}}.$$

Theorem: the group $H^1_{dR}(G)$ has dimension $h$, the height of $G$.

Question: where can I find a proof of this?

The above definitions and theorem are in pages 633-634 of Colmez' "Periodes $p$-adiques des varietes abeliennes" for example, and he refers to Fontaine's book "Groupes $p$-divisibles sur les corps locaux", but without giving a precise reference. Iovita also uses these definitions in "Formal sections and de Rham cohomology of semistable abelian varieties" and refers to chapter V of Katz' "Crystalline cohomology, Dieudonne modules and Jacobi sums". In either case, I can't say that the references have been very helpful.