Most of the countries which have a '6 month validity' rule do so to endure that (even if, say, you were hospitalised during your stay) your passport would remain valid until you leave the country. However EU citizens are NEVER obliged to leave any EU country. (You've got the right to stay for the rest of your life if you so choose). So there can be no 'minimum validity' rules for travel within the EU.
All EU member states are obliged to accept any EU passport up until the very last second of its validity (although, of course, it could cause a few problems if your passport actually expired during your stay in that country and you were unable to renew it).
Since you've asked for a link to an 'official' website, I'll give you this one:
http://www.fco.gov.uk...el-advice-by-country/
That site gives you the entry requirements, for British passport holders, for every country in the world. Here's the relevant information for Spain:
http://www.fco.gov.uk...ntryRequirements&pg=4
Chris