  Clouds of ultralight bosons - such as axions - can form around a rapidly
spinning black hole, if the black hole radius is comparable to the bosons'
wavelength. The cloud rapidly extracts angular momentum from the black hole,
and reduces it to a characteristic value that depends on the boson's mass as
well as on the black hole mass and spin. Therefore, a measurement of a black
hole mass and spin can be used to reveal or exclude the existence of such
bosons. Using the black holes released by LIGO and Virgo in their GWTC-2, we
perform a simultaneous measurement of the black hole spin distribution at
formation and the mass of the scalar boson. We find that the data strongly
disfavors the existence of scalar bosons in the mass range between
$1.3\times10^{-13}~\mathrm{eV}$ and $2.7\times10^{-13}~\mathrm{eV}$ for a decay
constant $f_a\gtrsim 10^{14}~\mathrm{GeV}$. The statistical evidence is mostly
driven by the two {binary black holes} systems GW190412 and GW190517, which
host rapidly spinning black holes. The region where bosons are excluded narrows
down if these two systems merged shortly ($\sim 10^5$ years) after the black
holes formed.
