Internal resistance of signal source
The parallel resistance of both the source and the oscilloscope.

You have connected them in parallel, so the combined resistance is \$\frac{R_1R_2}{R_1+R_2}\$. The oscilloscope is a small capacitor, so its DC resistance goes toward infinity, and the total source resistance is given by the generator.

Obviously, that is for the static case only. If you generate a variable signal, your total resistance calculation becomes a lot more complex, and needs to include signal reflection, the wave impedance of the cables used etc.

This is where expensive oscilloscope probes come in -- these are optimized to generate as little change in the electrical properties of the device under test as possible. The closest you can get with a cable based solution is a T connector on the oscilloscope frontend, so the two points of impedance mismatch (cable split and oscilloscope input amplifier) are as close together as possible.