Leonhard Euler studied the function that is now known as the Riemann zeta function. I have not found the notation $\zeta$ in any of the works of any mathematicians prior to Bernhard Riemann's paper On the Number of Primes Less Than a Given Quantity dated November, 1859. Did Euler know the zeta function as the zeta function or did he use a different notation for it?

I am seeking a reference for the first use of the notation $\zeta$ for a function.

What I am getting at is this:

"Unfortunately, overshadowed by the complex version of the zeta function subsequently developed and used by Bernard Riemann, Euler's original real zeta function seems to have dropped out of sight in popular expositions of mathematics of late. With the hope of similarly inspiring another generation of future mathematicians, this month's column tries to rekindle interest in Euler's original and spectacular eighteenth century theorem." - Keith Devlin https://www.maa.org/external_archive/devlin/devlin_10_01.html

See, there are two zeta functions. So, which one do we denote by $\zeta$ and how do we denote the other?