summation - Proving this infinite sum of a product of three binomials: $sumlimits_{s}binom{n+s}{k+l}binom ksbinom ls=binom nkbinom nl$

Question: How do you prove$$\sum\limits_{s}\binom{n+s}{k+l}\binom ks\binom ls=\binom nk\binom nl$$

I'm just not sure where to begin. I tried writing both sides as the coefficient of $x^n$ of the expansion of a binomial. But obviously, that doesn't fit the right-hand side because it's the product of two binomials.

I'm guessing that we'll need the multinomial theorem. Is that correct? Do you have any other ideas?