Prove that for a natural number $n$ is true $1^3 + 2^3 +...+ n^3 = (1 + 2 +...+ n)^2$

Question

So I have to show that for all natural numbers $n$ is true

$$1^3 + 2^3 +...+ n^3 = (1 + 2 +...+ n)^2$$

For $n = 1$ I proved it and I came this far:

$$1^3 + 2^3 +...+ n^3 + (n + 1)^3 = (1 + 2 +...+ n)^2 + (n + 1)^3$$

I do not know how to prove that part. Any help?