This meditation comes from ideas in the Bahiya Sutta about a seeker who went to the Buddha for advice…
https://www.accesstoinsight.org/tipitaka/kn/ud/ud.1.10.irel.html
A third time Bahiya said to the Lord: “It is difficult to know for certain… Teach me Dhamma, Sugata, so that it will be for my good and happiness for a long time.”
“Herein, Bahiya, you should train yourself thus: ‘In the seen will be merely what is seen; in the heard will be merely what is heard; in the sensed will be merely what is sensed; in the cognized will be merely what is cognized.’ In this way you should train yourself, Bahiya.
“When, Bahiya, for you in the seen is merely what is seen… in the cognized is merely what is cognized, then, Bahiya, you will not be ‘with that.’ When, Bahiya, you are not ‘with that,’ then, Bahiya, you will not be ‘in that.’ When, Bahiya, you are not ‘in that,’ then, Bahiya, you will be neither here nor beyond nor in between the two. Just this is the end of suffering.”
Now through this brief Dhamma teaching of the Lord the mind of Bahiya of the Bark-cloth was immediately freed from the taints without grasping. Then the Lord, having instructed Bahiya with this brief instruction, went away.