On the Papal Approbation of Gregory XVI Regarding the Writings of St. Alphonsus Liguori

I have extracted the following from https://www.goodcatholicbooks.org/alphonsus/alphonsus-facts.html (Papal approval) ``No ecclesiastical writer has ever received more direct, positive and formal approbation than that accorded by the Holy See to the moral writings of this Doctor of the Church. While still alive, four Popes expressed their admiration of his prudent doctrine. (…) In 1831, Pope Gregory XVI enhanced this approbation when he decreed that professors of theology could safely teach any opinion of St. Alphonsus, and that confessors, without weighting reasons, could safely follow him... I have searched on and off for over a year to find the document containing Pope Gregory's aforesaid approbation---but I have not found it. If I had a title of the document, it would be much easier, but the author of the above paragraph does not provide it. QUESTION: Does anyone know which document of Pope Gregory XVI contains this approbation he bestowed on the writings of St. Alphonsus Liguori? Also, would such an approbation be equivalent to an *imprimatur*? Thank you.