Some sort of indulgence for the prayer of St. Thomas?

A long time ago my dad told me that if you pray the prayer of St. Thomas (“My Lord and my God”) when the priest raises the chalice at Mass (after saying “Do this in memory of me”) then you will get less time in purgatory (an indulgence). Is this right?

Yes, I think so. I found a site that says so, but it seems to be a heretical site (denies the post-VatII Church), so I can’t post the link in good conscience.

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