The group $G=\langle t,a \mid t^4=1, (t^{-1}at)^{-1} a (t^{-1} at)=a^2 \rangle$ is non-residually finite

Question

A group $G= \langle t,a,| t^4=1, (t^{-1}at)^{-1} a (t^{-1}a t)= a^2 \rangle$ ( I do not remember the reference) was shown as a non -residually finite group. I do not know how to prove it. I will be thankfull for any hints.