He who sings prays twice.

Not found in his writings. In his "Expositions on the Psalms" for psalm 72, he wrote, "Qui enim cantat laudem, non solum laudat, sed etiam hilariter laudat; qui cantat laudem, non solum cantat, sed et amat eum quem cantat." An English translation would be "For he who sings praise, does not only praise, but also praises joyously; he who sings praise, is not only singing, but also loving Him whom he is singing of."

The earliest occurrence on Google Books of "Qui cantat, orat bis" is on page 4 of the 1554 book Cantiones Evangelicae: Ad Usitatas Harmonias... by Wenzel Nicolaides.