Source identified1
“When angry, count ten, before you speak; if very angry, an hundred.”
Jefferson’s practical rule inserts a deliberate delay between anger and speech, giving judgment time to return before emotion becomes an irreversible response.
Source note: Thomas Jefferson to Thomas Jefferson Smith, February 21, 1825, Founders Online / National Archives. The sentence is item 10 in a short list of “canons of conduct” sent to his namesake relative; the archive currently labels the transcription an Early Access document rather than a final authoritative edition.