Anxiety is the greatest evil that can befall a soul, except sin. God commands you to to pray, but He forbids you to worry. — Attr. St. Francis de Sales
The first sentence of the quotation is authentically St. Francis de Sales, from Introduction to the Devout Life, Part IV, Chapter 11. (Almost authentically; it’s been shortened and meme-ized.) But the second sentence of the alleged quotation isn’t from Devout Life; in fact, it doesn’t come from St. Francis de Sales at all. Google will quickly tell you that it comes from St. John Vianney, but usually without a reference. My friend Fr. Jerabek found the reference for me; it’s from one of the Curé’s homilies on refraining from Sunday labor. (“Il vous commande la prière, mais il vous défend l’inquiétude.”)
I was curious to see if I could tell when the two quotations were joined together. The earliest conjunction I have found is from 2009, when it appeared on EWTN’s website.