Relata refero ("I tell as I was told", or "I report reports") — Georg Büchmann, in Geflügelte Worte, 20th edn., p. 367, traces this phrase back to Herodotus, vii. 152: ἐγὼ δὲ ὀφείλω λέγειν τὰ λεγόμενα, πείθεσθαί γε μὲν οὐ παντάπασιν ὀφείλω καί μοι τοῦτο τὸ ἔπος ἐχέτω ἐς πάντα τὸν λόγον· ἐπεὶ καὶ ταῦτα λέγεται.
Relata is the plural past participle neutered and refero the first person singular, present indicative active of the verb conjugated refero, referre, retuli, relatum, the root of such words as 'relating', and 'reference'; the phrase is thus a polyptoton.
It was the motto of the pseudo-official Ordentliche Wochentliche Kayserliche Reichs-Post-Zeitungen, published from the imperial city of Frankfurt am Main, and called by Johann Georg Hamann 'the postilion of the Holy Roman Empire'.
The German Wikipedia focuses on the use of the phrase to dissociate its user from judging the content of his words; the Italian Wikipedia mentions, along with a similar statement of the responsibility to faithfully report, its uses in the continental legal system.