  We report the detection of a single burst from the first-discovered repeating
Fast Radio Burst source, FRB 121102, with CHIME/FRB, which operates in the
frequency band 400-800 MHz. The detected burst occurred on 2018 November 19 and
its emission extends down to at least 600 MHz, the lowest frequency detection
of this source yet. The burst, detected with a significance of 23.7$\sigma$,
has fluence 12$\pm$3 Jy ms and shows complex time and frequency morphology. The
34 ms width of the burst is the largest seen for this object at any frequency.
We find evidence of sub-burst structure that drifts downward in frequency at a
rate of -3.9$\pm$0.2 MHz ms$^{-1}$. Our best fit tentatively suggests a
dispersion measure of 563.6$\pm$0.5 pc cm$^{-3}$, which is ${\approx}$1% higher
than previously measured values. We set an upper limit on the scattering time
at 500 MHz of 9.6 ms, which is consistent with expectations from the
extrapolation from higher frequency data. We have exposure to the position of
FRB 121102 for a total of 11.3 hrs within the FWHM of the synthesized beams at
600 MHz from 2018 July 25 to 2019 February 25. We estimate on the basis of this
single event an average burst rate for FRB 121102 of 0.1-10 per day in the
400-800 MHz band for a median fluence threshold of 7 Jy ms in the stated time
interval.
