Deep ALMA and HST observations reveal the presence of a quenched massive galaxy within the $z=2.91$ galaxy group RO-1001. With a mass-weighted stellar age of $1.6 pm 0.4 ,$Gyr this galaxy is one of the oldest known at $zsim3$, implying that most of its $10^{11}rm , M_{odot}$ of stars were rapidly formed at $z>6$--8. This is a unique example of the predominantly passive evolution of a galaxy over at least $3<z<6$ following its high-redshift quenching and a smoking-gun event pointing to the early imprint of an age-environment relation. At the same time, being in a dense group environment with extensive cold-gas reservoirs as betrayed by a giant Ly$alpha$ halo, the existence of this galaxy demonstrates that gas accretion shutdown is not necessary for quenching and its maintenance.
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