We report the detection of the $^2\rm P_{3/2} \rightarrow {^2P_{1/2}}$ fine-structure line of $\rm C^+$ at 157.74 $\rm \mu m$ in SDSS J114816.64+525150.3 (hereafter J1148+5251), the most distant known quasar, at $z=6.42$, using the IRAM 30-m telescope. This is the first detection of the Cii line at high redshift, and also the first detection in a Hyperluminous Infrared Galaxy ($L_{\rm FIR} > 10^{13} \, L_\odot$). The Cii line is detected at a significance level of 8σ and has a luminosity of $ 4.4\times 10^9 \,L_\odot$. The $L_{\rm CII}/L_{\rm FIR}$ ratio is $\rm 2\times 10^{-4}$, about an order of magnitude smaller than observed in local normal galaxies and similar to the ratio observed in local Ultraluminous Infrared Galaxies. The Cii line luminosity indicates that the host galaxy of this quasar is undergoing an intense burst of star formation with an estimated rate of ≈$ 3000 \, M_\odot \, \rm yr^{-1}$. The detection of $\rm C^+$ in SDSS J1148+5251 suggests a significant enrichment of metals at $z\sim 6$ (age of the universe ~870 Myr), although the data are consistent with a reduced carbon to oxygen ratio as expected from chemical evolutionary models of the early phases of galaxy formation.