  Fluorine is one of the most interesting elements for nuclear and stellar
astrophysics. Fluorine abundance was first measured for stars other than the
Sun in 1992, then for a handful metal-poor stars, which are likely to have
formed in the early Universe. The main production sites of fluorine are under
debate and include asymptotic giant branch (AGB) stars, $\nu$-process in
core-collapse supernovae, and Wolf-Rayet (WR) stars. Due to the difference in
the mass and lifetime of progenitor stars, high redshift observations of
fluorine can help constrain the mechanism of fluorine production in massive
galaxies. Here, we report the detection of HF (S/N = 8) in absorption in a
gravitationally lensed dusty star-forming galaxy at redshift z=4.4 with $N_{\rm
HF}$/$N_{\rm{H_2}}$ as high as $\sim2\times10^{-9}$, indicating a very quick
ramp-up of the chemical enrichment in this high-z galaxy. At z=4.4, AGB stars
of a few solar masses are very unlikely to dominate the enrichment. Instead, we
show that WR stars are required to produce the observed fluorine abundance at
this time, with other production mechanisms becoming important at later times.
These observations therefore provide an insight into the underlying processes
driving the `ramp-up' phase of chemical enrichment alongside rapid stellar mass
assembly in a young massive galaxy.
