Thallus on the passion phenomena.

As cited by Julius Africanus, as cited by George Syncellus.

One of the ancient (probably) pagan testimonia. Also refer to the Phlegon testimonium.

There is a solid discussion of the following texts by Richard Carrier available online at the Internet Infidels. Carrier also helpfully translates (and comments on) Felix Jacoby, Die Fragmente der griechischen Historiker, compiling ancient references to the history of Thallus, which is unfortunately lost to us. A .pdf file of the fragments of Thallus is available from Christian Hospitality.

George Syncellus, century IX, Chronography chapter 391, citing Julius Africanus, early century III (Greek text from Felix Jacoby, Die Fragmente der griechischen Historiker, page 1157, Thallus history 1, and page 1165, Phlegon history 16b; English translation based on the online excerpt at the Tertullian Project):

Καθ ολου του κοσμου σκοτος επηγετο φοβερωτατον, σεισμω τε αι πετραι διερρηγνυντο και τα πολλα Ιουδαιας και της λοιπης γης κατερριφθη. τουτο το σκοτος εκλειψιν του ηλιου Θαλλος αποκαλει εν τριτη των ιστοριων, ως εμοι δοκει, αλογως. Εβραιοι γαρ αγουσι το πασχα κατα σεληνην ι̅δ̅, προ δε μιας του πασχα τα περι τον σωτηρα συμβαινει. εκλειψις δε ηλιου σεληνης υπελθουσης τον ηλιον γινεται· αδυνατον δε εν αλλω χρονω, πλην εν τω μεταξυ μιας και της προ αυτης κατα την συνοδον αυτην αποβηναι. πως ουν εκλειψις νομισθειη κατα διαμετρον σχεδον υπαρχουσης της σεληνης ηλιω; εστω δη, συναρπαζετω τους πολλους το γεγενημενον και το κοσμικον τερας ηλιου εκλειψις υπονοεισθω εν τη κατα την οψιν. Φλεγων ιστορει επι Τιβεριου Καισαρος εν πανσεληνω εκλειψιν ηλιου γεγονεναι τελειαν απο ωρας εκτης μεχρις ενατης, δηλον ως ταυτην. τις δ η κοινωνια σεισμω και εκλειψει, πετραις ρηγνυμεναις, και αναστασει νεκρων τοσαυτη τε κινησει κοσμικη;

A most terrible darkness fell over all the world, the rocks were torn apart by an earthquake, and many places both in Judaea and the rest of the world were thrown down. In the third book of his Histories Thallus dismisses this darkness as a solar eclipse, unreasonably, as it seems to me. For the Hebrews celebrate the Passover on Luna 14, and what happened to the Saviour occurred one day before the Passover. But an eclipse of the sun takes place when the moon passes under the sun. The only time when this can happen is in the interval between the first day of the new moon and the last day of the old moon, when they are in conjunction. How then could one believe an eclipse took place when the moon was almost in opposition to the sun? So be it. Let what had happened beguile the masses, and let this wonderful sign to the world be considered a solar eclipse through an optical [illusion]. Phlegon records that during the reign of Tiberius Caesar there was a complete solar eclipse at full moon from the sixth to the ninth hour; it is clear that this is the one. But what have eclipses to do with an earthquake, rocks breaking apart, resurrection of the dead, and a universal disturbance of this nature

Carrier mentions a passage from Josephus the conjectured restoration of which is sometimes made to point to Thallus. For convenience I offer that passage here. Antiquities 18.6.4 §167:

Και γαρ ην αλλος,* Σαμαρευς γενος, Καισαρος δε απελευθερος· παρα τουτου δανεισμα μυριαδας εκατον ευρομενος τη τε Αντωνια καταβαλλει το οφειληθεν χρεος και των λοιπων τω αναλωματι θεραπευων τον Γαιον μειζονως εν αξιωματι ην παρ αυτω.

* Sometimes conjectured as Θαλλος (Thallus).

For there was also another, Samaritan by race, and a freeman of Caesar; from this man he borrowed a million drachmae, and from this he repaid Antonia the debt he owed her, and by sending the surplus in paying his court to Gaius became a person of great worthiness with him.