In the Bible. It is Exodus 4:13, and it is Moses's last try at turning the job down. He has already said he is "slow of speech and slow of tongue" (Exodus 4:10). The next verse says "the anger of the Lord was kindled against Moses" (Exodus 4:14). God does not take the job back. He gives Moses his brother Aaron to do the talking (Exodus 4:14-16).
Moses raises four problems at the bush before this line. He asks who he is to go, then what God's name is, then says the people will not believe him, then says he is a poor speaker. The New Oxford Annotated Bible, a study Bible, says that last one may mean a speech impediment, or may just be a prophet trying to get out of the job.
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