"No man is fit to command another that cannot command himself." -- attributed to William Penn (b. 1644-10-14, d. 1718-07-30), but none of the citations I've found identify the work or letter it's from and it's not on his Wikiquote page. Using it here anyhow on account of Trump.
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Actually, I think I care less about whether Penn said it than about knowing for sure who said it, whoever that is. But it would be a litte less messy if the many, many sites attributing it to him turn out to be right.