If $G$ is any group and $\gamma_k(G)$ denotes the $k$th term in the lower central series of $G$, then the commutator bracket on $G$ endows

$$\mathcal{L}(G) = \bigoplus_{k=1}^{\infty} \gamma_k(G) / \gamma_{k+1}(G)$$

with the structure of a Lie ring. A famous theorem that should probably be attributed to Magnus says that if $G$ is the free group on $n$ generators, then $\mathcal{L}(G)$ is the free Lie ring on $n$ generators. This result is documented in many places, e.g. Magnus--Karass--Solitar's book on combinatorial group theory and Serre's book Lie Group and Lie Algebras.

Now fix a prime $p$ and let $\delta_k(G)$ be the fastest descending central series of $G$ satisfying the following three properties:

$\delta_1(G) = G$, and

$[\delta_k(G),\delta_{\ell}(G)] \subset \delta_{k+\ell}(G)$ for all $k,\ell \geq 1$, and

$(\delta_k(G))^p \subset \delta_{pk}(G)$ for all $k \geq 1$.

This series was first defined by Zassenhaus. The commutator bracket and the $p$th power operation on $G$ endow

$$\Lambda(G) = \bigoplus_{k=1}^{\infty} \delta_k(G) / \delta_{k+1}(G)$$

with the structure of a restricted Lie algebra over the field $\mathbb{F}_p$.

I'm pretty certain that I can prove that if $G$ is a free group on $n$ generators, then $\Lambda(G)$ is the free restricted Lie algebra on $n$ generators over the field $\mathbb{F}_p$. I need this result for a paper I am writing (on an unrelated topic), but I'm certain that this result is known and would greatly prefer to just cite a reference for it.

**Question**: Can anyone give me a reference for this?