Reference Cells on same workbook - different sheet

Reference Cells on same workbook - different sheet

Hi,

I am having problems referencing cells on the same workbook on a different sheet. Can you confirm this is the correct syntax please:

=.Tests:C91 where Tests is the name of the sheet and cell C91 is what I want to display. There is a number in cell C91 on sheet Tests of 666. I get an error message: #invalid formular!

Hope you can help!

Thanks

John