While not St. Francis of Assissi's, it glows with his spirit. As for Guilluame, the Harrowing of the North doesn't quite strike me as "sowing peace" but so it is for warriors who must cede personal beliefs to the job at hand.
English translation- the original is French-
The Peace Prayer of Saint Francis
O Lord, make me an instrument of Thy Peace!Where there is hatred, let me sow love.
Where is discord, harmony.Where there is injury, pardon.
Where there is doubt, faith.
Where there is despair, hope.
Where there is darkness, light.
Where there is sorrow, joy
O, Divine Master, grant that I may not so much seek to be consoled as to console;to be understood as to understand; to be loved as to love;for it is in giving that we receiveit is in pardoning that we are pardoned; and it is in dying that we gain eternal life.