Its a paraphrase of DN1, DN2 and DN9.
A “real” existant could not “depend” on variables like time or space or a part-whole relationship without the “real” existant being conditional on those things. A “really” real thing should exist “unconditionally”.
Basically the buddha critiqued the idea that there could be things that simply “exist” as “reals” in a niave way.
As I say, see for starters DN1 DN2 and DN9, then for furthur reading see my thread here:
For the most famous articulation of this thesis outside the ebt see Nagarjuna’s Mulamadyamikakarika, I recommend Jones, but there are plenty of translations around.
For some reading about the broader issues involved I highly recommend the Oxford handbook of Metaphysics.
Good luck!