Tax at what rate

Respected Sir & Colleagues
I hv came across one problem plz do the need to solve it, my problem is here under

If suppose any service provider renders his services in m/o feb & march 06, but he raised bill to his client in may06.
My Quirey is at what rate he will charge service tax?

At the rate of 10 percent-  rate at the time when service was provided.
OR
At the rate of 12 percent- rate at the time of raising bill.
Plz provide your opinion based on relevant section of the Act.

Thanks & Regards