We have a website hosted behind WAF and we will be using HTTPS. The server already has a valid SSL. Do we need to install SSL on WAF servers also?
A WAF applies filter rules on traffic at an "application" level (e.g. it tries to detect SQL injection attempts). This requires that the WAF sees the traffic, i.e. whatever SSL which may have happened on the client side must stop at the WAF. But you usually want some SSL to protect the traffic between the client and the WAF (in fact, you usually want it more on that link than between the WAF and the server itself, since WAF and server are usually nearby to each other).
There are two ways for a WAF to see SSL-protected traffic:
The WAF has a copy of the private key used by the (SSL-aware) server, and thus can decrypt the data as it flows. (This can imply some restrictions on the cipher suites used by the server; namely, no DHE).
The WAF itself runs a SSL server, and that is the one which the client sees. The WAF decrypt the data, runs its magic on it, and then forwards it to the server over a new connection which may or may not be SSL-protected.
Which one you use depends on what your WAF instance can do and how you configured it.
Either way, the SSL certificate will need to be installed on the WAF in order for it to inspect the encrypted traffic.