Cross reference
Exodus 6:30
30 And Moses said to the Lord, My lips are unclean; how is it possible that Pharaoh will give me a hearing?
Exodus 6:12
12 And Moses, answering the Lord, said, See, the children of Israel will not give ear to me; how then will Pharaoh give ear to me, whose lips are unclean?
Exodus 4:10
10 And Moses said to the Lord, O Lord, I am not a man of words; I have never been so, and am not now, even after what you have said to your servant: for talking is hard for me, and I am slow of tongue.
1 Corinthians 9:16-17
16 For if I am a preacher of the good news, I have no cause for pride in this; because I am forced to do so, for a curse is on me if I do not.
17 But if I do it gladly, I have a reward; and if not, I am under orders to do it.