E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Moses Slow of Speech.
The account given in the Talmud (vi.) is as follows:Pharaoh was one day sitting on his throne with Moses on his lap, when the child took off the kings crown and put it on his own head. The wise men tried to persuade the king that this was treason, for which the child ought to be put to death; but Jethro, priest of Midian, replied, It is the act of a child who knows no better. Let two plates (he continued) be set before him, one containing gold and the other red-hot coals, and you will readily see he will prefer the latter to the former. The experiment being tried, the little boy snatched up the live coal, put it into his mouth, and burnt his tongue so severely that he was ever after heavy or slow of speech.