Do you have to measure mains voltage waveform just once in your whole life? Differential probes are too expensive? Here it is a simple, cheap, DIY solution.
CAUTION: Working on mains voltage is dangerous, you MUST know what you do.
Directly using the probes of the oscilloscope to measure the mains is hazardous, it could lead to from destroying your probes to melting the traces of the PCB inside your oscilloscope, not to mention the risk of being injured. Depending by the grid system you may have 50% to 100% chance of causing a short capable to destroy your instrumentation.
In an old video I presented a depth analysis of why you can blow up your oscilloscope while making measurements at mains, follow this link:    • Correct use of the oscilloscope when ...  
Also the trick that uses two probes directly may end up in catastrophic results if you work on voltages above 110V RMS.
In this video is presented a simple attenuator made of just 1/4W resistors, that guarantees safe operations with the oscilloscope provided the PE (Protective Earth, the yellowgreen/green wire) is actually tied to ground. It still requires two input channels of your oscilloscope but it is safer than directly hooking the probes to the mains.

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