Old English secgan "to utter, inform, speak, tell, relate," from Proto-Germanic *sagjanan (source also of Old Saxon seggian, Old Norse segja, Danish sige, Old Frisian sedsa, Middle Dutch segghen, Dutch zeggen, Old High German sagen, German sagen "to say"), from PIE *sokwyo-, from root *sekw- (3) "to say, utter" (source also of Hittite shakiya- "to declare," Lithuanian sakyti "to say," Old Church Slavonic sociti "to vindicate, show," Old Irish insce "speech," Old Latin inseque "to tell say").
Past tense said developed from Old English segde. Not attested in use with inanimate objects (clocks, signs, etc.) as subjects before 1930. You said it "you're right" first recorded 1919; you can say that again as a phrase expressing agreement is recorded from 1942, American English. You don't say (so) as an expression of astonishment (often ironic) is first recorded 1779, American English.
"what someone says," 1570s, from say (v.). Meaning "right or authority to influence a decision" is from 1610s. Extended form say-so is first recorded 1630s. Compare Old English secge "speech."