A quote often attributed to St. Francis doesn't seem to have a credible source, but it's still a good saying, and others appear to have said similarly.
One of the last things we did at the seminar with Bernard McGinn this past weekend was read through and discuss Saint Francis of Assisi's poem and religious song Canticle of the Sun. McGinn considers this to be a very mystical text from Francis, as Francis seems to see God powerfully in and throughout the whole of creation, including in the sun, moon, stars, Earth, etc. McGinn noted that it is a kind of nature mysticism. Francis wrote most of it in the year 1224, and the last few lines in 1226 just before his death.