Who introduced the comma notation for partial derivatives?

In general relativity, it is common to use the comma notation for partial derivatives

$$frac{partial g_{munu}}{partial x_rho} = g_{munu_,rho}$$

Where did this notation first appear? Was it known in general mathematics before Einstein?

I don’t know much about mathematics books, but I traced it back to Synge’s book on General Relativity(1960). Could it have arisen earlier?