God has entrusted the keys and treasures of the Kingdom of Heaven to Mary.
Attr. to St. Thomas Aquinas
I found a site (connected with a disapproved apparition, so I won’t give a link) which claims it comes from St. Thomas’s “Exposition on the Salve Regina.” The problem with that is that he didn’t write one. He did write on the Angelic Salutation, but the alleged quotation is not in there either.
The friend who asked me about this suggested that I try St. Alphonsus Ligouri’s The Glories of Mary, and behold, it’s there … attributed to Louis de Blois, better known as Blosius. The editor of the edition I found even gives a citation, which alas is not entirely legible, though you’re welcome to try for yourself here. But since there is a citation to a definite work, I’m going to attribute this one to Blosius (and wonder how it got attributed to St. Thomas).
Edit: Thanks to Andrew in the comments below, I have a Google Books link to the saying from Brosius in Latin, As he notes in his comment, the quotation here doesn’t mention the keys:
Tibi regni cæleſis theſauri commiſſi ſunt. (To you [i.e., Mary] the treasures of heaven have been entrusted.)
(I am very disappointed that I couldn’t enter a double long-s.)