passive probes and accurate measurements
The rule of thumb is 5x for 2% down (plus whatever inaccuracy there is in the 'scope circuitry since you are talking about total measurement accuracy). 

At the rated frequency for the 'scope + probe your signal will be reduced to 70% in voltage, so it's the -3dB or half-power point. 

Note that is for sine waves only. If your signal is anything but a sine wave (perhaps + DC) you will need more bandwidth, if there are sharp edges and you want to see them accurately, much more bandwidth. Another rule of thumb is that the bandwidth in GHz is 0.35/RT, where RT is the rise time in nanoseconds. So a 100MHz square wave with rise time of 500ps has bandwidth of 700MHz, and if you really wanted to measure the rise time accurately you should have a 'scope bandwidth in the GHz. A 100MHz scope could show something like a reduced amplitude and distorted sine wave and meet specifications. What it actually shows depends on the scope design and any anti-aliasing filter they have implemented, assuming a digital scope. 

Keep in mind that the probes may present significant loading on the source. Using 10x probes typically results in much better frequency response and much less loading at moderate frequencies, but the noise floor is going to be 10x worse. 

Image from Tektronix link above.