Most password crackers can do that, but depending on the length of the number or whether it is just a number or also contains letters, etc, it'll take you years to do it (if it's just a few numbers, it'll take you seconds instead).
Just google MD5 cracker or MD5 decrypter, or something similar, and you should get tons of possibilities.
Start by trying this online service:
It'll just compare your hash to a huge online database, so if your source is already in their database, it'll be quick.
But check your hash here:
- pretty sure it's not an MD5 hash.