While scholars debate whether the beloved Peace Prayer which bears his name was actually composed by Francis or not (seems it's hard to trace back before 1912), there's another prayer which we can be pretty sure he uttered before the crucifix of San Damiano--the ancient cross from which Christ famously spoke and gave Francis his mission: Rebuild my Church...
Most High, Glorious God,
enlighten the darkness of my heart.
Give me right faith, sure hope,
and perfect charity.
Fill me with understanding and knowledge
that I may fulfill your holy and true command.