Perform restore without update?

I want to restore my iPad to factory settings. Its an iPad 2,1 running iOS 8.1:

However, when I connect to iTunes and click Restore iPad..., I get the message:

I'm not interested in 8.1.3, which is what a Restore and Update would result. In fact, I need to keep the iPad at 8.1.2 or lower due to operational requirements. So iOS 8.1 is fine by me.

How do I perform a restore without updating?