Attaching a resistor to a oscilliscope probe
As an alternative to doing this, you might want to consider making an active voltage follower using a low Ib op-amp. 

Even an inexpensive TL071 will have something like 1T ohm input resistance and input capacitance of  if you stay inside the input voltage range (eg. give it something like +/-10V supplies and stay within +/-8V. Bias current is under 100pA typically at room temperature so 10M ohm source resistance will contribute negligible error (in comparison to the offset voltage). There are much lower Ib op-amps available (down to fA).