Old English soð "truth, justice, righteousness, rectitude; reality, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth").
The group is related to Old English synn "sin" and Latin sontis "guilty" (truth is related to guilt via "being the one;" see sin (v.)), from PIE *hes-ont- "being, existence," thus "real, true" (from present participle of root *es- "to be"), also preserved in Latin sunt "they are" and German sind.
Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). In common use until mid-17c., then obsolete until revived as an archaism early 19c. by Scott, etc. Used for Latin pro- in translating compounds into Old English, such as soðtacen "prodigy," soðfylgan "prosequi."
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.