Here is a direct proof for free groups.

Let $x_1,\dots,x_m$ be the generators of our group. Consider a word $x_{i_n}^{e_n}\dots x_{i_2}^{e_2}x_{i_1}^{e_1}$ where $e_i\in\{\pm 1\}$ and there are no cancellations (that is, $e_k=e_{k+1}$ if $i_k=i_{k+1}$).

I'm going to map this word to a nontrivial element of $S_{n+1}$, the group of permutations of $M:=\{1,\dots,n+1\}$. It suffices to construct permutations $f_1,\dots,f_m\in S_{n+1}$ such that $f_{i_n}^{e_n}\dots f_{i_2}^{e_2}f_{i_1}^{e_1}\ne id_M$. For each $k=1,\dots,n$, assign $f_{i_k}(k)=k+1$ if $e_k=1$, or $f_{i_k}(k+1)=k$ if $e_k=-1$. This gives us injective maps $f_1,\dots,f_m$ defined on subsets of $M$. Assign yet unassigned values of $f_i$'s arbitrarily (the only requirement is that they are bijections). The resulting permutations satisfy $f_{i_n}^{e_n}\dots f_{i_2}^{e_2}f_{i_1}^{e_1}(1)=n+1$.

**Edit**: As pointed out by Steve D in comments, this proof can be found in a book by Daniel E. Cohen, "Combinatorial group theory: a topological approach" (1989). The book can be found on the net if you are determined; the proof in on page 7 and in Proposition 5 on page 11.

Edit [DZ]: I have a hard time reading multiple subscripts, so here is an example of Sergei Ivanov's construction.

Take the word $cca^{-1}bc^{-1}a$. This has length $6$, so we will find a homomorphism to $S_7$ whose image of this word is nontrivial because it sends $1$ to $7$. We'll choose values of permutations so that the $k$th suffix sends $1$ to $k+1$:

Suffix 1: $a$

$\alpha=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ 2&?&?&?&?&?&? \end{array} \bigg)$

Suffix 2: $c^{-1}a$

$\gamma=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ ?&?&2&?&?&?&? \end{array} \bigg)$

Suffix 3: $bc^{-1}a$

$\beta=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ ?&?&4&?&?&?&? \end{array} \bigg)$

Suffix 4: $a^{-1}bc^{-1}a$

$\alpha=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ 2&?&?&?&4&?&? \end{array} \bigg)$

Suffix 5: $ca^{-1}bc^{-1}a$

$\gamma=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ ?&?&2&?&6&?&? \end{array} \bigg)$

Suffix 6: $cca^{-1}bc^{-1}a$

$\gamma=\bigg(\begin{array}{} 1&2 &3 &4& 5& 6& 7 \\\ ?&?&2&?&6&7&? \end{array} \bigg)$

These conditions on $\alpha, \beta,$ and $\gamma$ don't conflict, they can be extended to permutations, and then $\gamma\gamma\alpha^{-1}\beta\gamma^{-1}\alpha(1) = 7$.