Don't use the built in Tab control.  It's not that good.  Only one tab's worth of contents is available to the browser at a some.  Changing tabs causes a round trip to the server.  Yuk!

 

The the one from the ext. library or just use manual dojo.   

selecte the tab on click of which it is giving you validation msg. and on the bottom right there is one check box called Do no validate or update data check this so that it will not validate you data when you switch the tab in tabtable.