Can somebody tell me or give a link to how to sign a message from the bitcoin address that I used to pay?
it's possible through blockchain.info, bitcoin official wallet and electrum afaik.
i used blockchain.info.
So you'd have to dumpprivkey from your wallet, then import the key to blockchain.info wallet. then on homepage of wallet, (right after logging in) you click on the desire address then select "sign message"
Blockchain.info is quite safe, i've been using it for 6 months now. But always be sure that your privkey is safe, and there's no trojan, or keyloggers on your pc. then you're good to go. Plus make sure your password on blockchain.info is different than the one on bitcointalk just in case.
You can use the vanilla Bitcoin Core wallet, and it has the feature.
I am so glad I got my stake barely last week.