iTunes backup problem

Hi there

First of all I'd like to apologize for my english but this forum is my last hope.
I was trying to create backup of iPad gen.3 on a new installation of mac os x 10.9.3 but I couldn't because iTunes asked me about password. I do not remember any passwords. Password never been stored in Keychain Access and computer is without any previous backups. I checked on two different computers and I had the same situation. I have no idea where this password is stored. It seems that is stored on the iPad but where? I've never heard about such functionality.
Thank you in advance for your help.

Weaselboy said:

I believe this would be the password attached to the AppleID you used in iTunes to purchase apps.

https://iforgot.apple.com/password/verify/appleid

You can go here to have Apple email you the password if you forgot it.

Click to expand...

I didn't use iCloud remote backup for this iPad. iTunes is asking about password for local backup that does not exist.