  The $\gamma$-ray emission from molecular clouds is widely believed to have a
hadronic origin, while unequivocal evidence is still lacking. In this work, we
analyze the Fermi-LAT Pass 8 publicly available data accumulated from 2008
August 4 to 2017 June 29 and report the significant detection of a
characteristic $\pi^0$-decay feature in the $\gamma$-ray spectra of the
molecular clouds Orion A and Orion B. This detection provides a direct evidence
for the hadronic origin of their $\gamma$-ray emission.
