Greetings! @TripleGemStudent,
It’s mentioned that any lay person who attained Arahanthood must ordain as a bhikkhu within 7 days.
The below text is in a book:
In the fifth year after Enlightenment, when the Buddha was residing in the (kutagara) Pinnacle Hall near Vesali, he heard that king Suddhodana was nearing death. He went to the death chamber and delivered a sermon. Having listened to it the king attained Arahantship. As he was too old, he did not join the order but enjoyed the bliss of Emancipation for seven days and passed away, (if a person attains the final stage of Sainthood that person cannot remain a lay person but either pass away or join the order).
This maybe the text which gives the idea that any lay person who attained Arahanthood must ordain as a bhikkhu within 7 days.
With mettā, Seng Kiat