O God Almighty,
who cleansed the lips of the prophet Isaiah with a burning coal:
Cleanse my heart and my lips.
So grant to cleanse me, of your mercy,
that I may be able to proclaim worthily your holy gospel:
through Jesus Christ our Lord. Amen.
Source: Ambrose, d. 397
Source of this version: https://blogs.thegospelcoalition.org/trevinwax/2007/07/29/cleanse-me-so-i-can-preach/
Also quoted in The One Year Book of Personal Prayer, © 1991 Tyndale House Publishers (April 14)
The burning coal is a reference to Isaiah 6:6-7
A Collection of Prayers 