It says this in MN 22 :
And how is a mendicant a noble one with banner and burden put down, detached? It’s when a mendicant has given up the conceit ‘I am’, cut it off at the root, made it like a palm stump, obliterated it, so it’s unable to arise in the future. That’s how a mendicant is a noble one with banner and burden put down, detached.
When a mendicant’s mind was freed like this, the gods together with Indra, Brahmā, and Pajāpati, search as they may, will not discover: ‘This is what the Realized One’s consciousness depends on.’ Why is that? Because even in the present life the Realized One is not found, I say.
And this is a (commentary) note on the passage by Nyanaponika Thera from this translation:
Ditth’ev’aaha.m bhikkhave dhamme Tathaagata.m ananuve jjo’ti vadaami. Comy: The term tathaagato (lit.: “thus-gone”) may refer either to a being (satto ) or to the greatest man (uttamo puriso ; the Buddha) and a taint-free saint (khiinaasavo ). Ananuve jjo means either “non-existing” (asa.mvijjamaano ) or “not traceable” (avindeyyo ). If tathaagato is taken as “a being” (in the sense of an abiding personality), the meaning “non-existing” applies; if in the sense of a taint-free saint, the meaning “not traceable” is apt. The intention implied in the first case, is: “O bhikkhus, even of a taint-free saint during his lifetime, here and now, I do not declare that he is ‘a being, a personality’ (in the sense of an abiding entity); how, then, should I declare it of a taint-free saint who has finally passed away, without any future rebirth? One thus-gone is untraceable; because in the ultimate sense (paramatthato ), there is no such thing as ‘a being’ (satto ). Searching for the basis of consciousness of such a non-existing (being) how can they find it, how can they obtain it?” In the case of the second explanation, the intention is this: “I say that Indra and other gods cannot trace a taint-free saint by way of consciousness (viññaanavasena ). For the gods who are with Indra and other deities, even if they make a search, cannot know about the consciousness of insight or that of the supramundane path or fruition (of sainthood; arahatta ) that ‘it proceeds based on such or such an object.’ How, then, could they know it in the case of one who has finally passed away (parinibbuto ), and has not been born again?” [Sub-Comy: "The consciousness of insight (vipassanaa-citta ) that aims at the attainment of the highest fruition (i.e., arahatta ) leaps forward to the unconditioned element (Nibbaana ) in the thought: “Non-origination is safety. Non-origination is safety!”]