How can I find what was the initial hash number ...
There's no such thing. Every miner assembles their own choice of transactions and makes their own choice of initial nonce value. There's no way to find out which combinations and values each miner tried first - they simply discard ones that don't hash less than the target, there's no record of failed attempts.
... and prove of work?
The hash being less than the target appropriate for that block height is the proof of work. Your example number is the proof of work for that block.
To validate that proof you just need to recalculate the hash and see that it agrees with the one you posted and then calculate the current network target for that block height based on prior block targets and times.
Useful links
