Is there a rule where if a monk is an arahant and if he is to tell he is an arahant, he can only say it to a Upasampada monk and not even a samanera monk?
If so, please provide the source of the rule
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According to the Patimokkha rules:
Should any bhikkhu, without direct knowledge, claim a superior human state, a truly noble knowledge and vision, as present in himself, saying, "Thus do I know; thus do I see," such that regardless of whether or not he is cross-examined on a later occasion, he — being remorseful and desirous of purification — might say, "Friends, not knowing, I said I know; not seeing, I said I see — vainly, falsely, idly," unless it was from over-estimation, he also is defeated and no longer in affiliation.
Should any bhikkhu report (his own) superior human state to an unordained person, when it is factual, it is to be confessed.
But above, there are no distinctions made between fully ordained monks and novice monks.
A monk can only relate his actual attainment of a superior human state to:
In other words, he commits an offense if he relates it to, e.g., a novice (samanera).
Since he told the truth, however, he won't be expelled — he's still considered a bhikkhu — but as @ruben2020 mentioned: he should confess the offense to another bhikkhu.
The factors for the full offense here are two:
Effort: One reports one’s actual attainment of a superior human state
Object: to an unordained person, i.e., any human being who is not a bhikkhu or bhikkhunī.