Livy 25.31.9-10 (capture of Syracuse, 212 BC; tr. J.C. Yardley):
 There were many instances of atrocities committed
from anger and from greed. Tradition has it that amidst the uproar,
such as the fear reigning in a captured city might arouse, with soldiers
running on the rampage everywhere, Archimedes was still concentrating intensely on some figures that he had drawn in the dust, and
was killed by a soldier who did not know who he was.  Marcellus, it is
said, was upset by this. He made careful arrangements for his
funeral, and also conducted a search for Archimedes' relatives, who
then received honour and protection, thanks to the man's reputation
 cum multa irae, multa avaritiae foeda exempla
ederentur, Archimeden memoriae proditum est in tanto tumultu, quantum captae <terror>
urbis in discursu diripientium militum ciere poterat, intentum formis
quas in pulvere descripserat ab ignaro milite quis esset interfectum;  aegre id
Marcellum tulisse sepulturaeque curam habitam, et propinquis etiam inquisitis
honori praesidioque nomen ac memoriam eius fuisse.
9 terror add. Böttcher: pavor add. Weissenborn
Cicero, De Finibus
5.19.50 (tr. H. Rackham):
What an ardour for study, think you, possessed Archimedes, who was so absorbed in a diagram
he was drawing in the dust that he was unaware even
of the capture of his native city!
quem enim ardorem studi censetis fuisse in
Archimede, qui dum in pulvere quaedam describit
attentius, ne patriam quidem captam esse senserit!
Pliny, Natural History
7.37.125 (tr. H. Rackham):
Archimedes also received striking testimony to his knowledge of geometry and mechanics from Marcus Marcellus, who at the capture of Syracuse forbade violence to be done to him only—had not
the ignorance of a soldier foiled the command.
grande et Archimedi geometricae ac machinalis scientiae testimonium M. Marcelli contigit interdicto cum Syracusae caperentur ne violaretur unus, nisi
fefellisset imperium militaris imprudentia.
Plutarch, Life of Marcellus
19.4-6 (tr. Bernadotte Perrin):
 But what most of all afflicted Marcellus was the death of Archimedes. For it chanced that he was by himself, working out some problem with the aid of a diagram, and having fixed his thoughts and his eyes as well upon the matter of his study, he was not aware of the incursion of the Romans or of the capture of the city. Suddenly a soldier came upon him and ordered him to go with him to Marcellus. This Archimedes refused to do until he had worked out his problem and established his demonstration, whereupon the soldier flew into a passion, drew his sword, and dispatched him.
 Others, however, say that the Roman came upon him with drawn sword threatening to kill him at once, and that Archimedes, when he saw him, earnestly besought him to wait a little while, that he might not leave the result that he was seeking incomplete and without demonstration; but the soldier paid no heed to him and made an end of him.
 There is also a third story, that as Archimedes was carrying to Marcellus some of his mathematical instruments, such as sun-dials and spheres and quadrants, by means of which he made the magnitude of the sun appreciable to the eye, some soldiers fell in with him, and thinking that he was carrying gold in the box, slew him. However, it is generally agreed that Marcellus was afflicted at his death, and turned away from his slayer as from a polluted person, and sought out the kindred of Archimedes and paid them honour.
 μάλιστα δὲ τὸ Ἀρχιμήδους πάθος ἠνίασε Μάρκελλον. ἔτυχε μὲν γὰρ αὐτός τι καθ᾽ ἑαυτὸν ἀνασκοπῶν ἐπὶ διαγράμματος· καὶ τῇ θεωρίᾳ δεδωκὼς ἅμα τήν τε διάνοιαν καὶ τήν πρόσοψιν οὐ προῄσθετο τήν καταδρομὴν τῶν Ῥωμαίων οὐδὲ τήν ἅλωσιν τῆς πόλεως, ἄφνω δὲ ἐπιστάντος αὐτῷ στρατιώτου καὶ κελεύοντος ἀκολουθεῖν πρὸς Μάρκελλον οὐκ ἐβούλετο πρὶν ἢ τελέσαι τὸ πρόβλημα καὶ καταστῆσαι πρὸς τήν ἀπόδειξιν.
 ὁ δὲ ὀργισθεὶς καὶ σπασάμενος τὸ ξίφος ἀνεῖλεν αὐτόν. ἕτεροι μὲν οὖν λέγουσιν ἐπιστῆναι μὲν εὐθὺς ὡς ἀποκτενοῦντα ξιφήρη τὸν Ῥωμαῖον, ἐκεῖνον δ᾽ ἰδόντα δεῖσθαι καὶ ἀντιβολεῖν ἀναμεῖναι βραχὺν χρόνον, ὡς μὴ καταλίπῃ τὸ ζητούμενον ἀτελὲς καὶ ἀθεώρητον, τὸν δὲ οὐ φροντίσαντα διαχρήσασθαι.
 καὶ τρίτος ἐστὶ λόγος, ὡς κομίζοντι πρὸς Μάρκελλον αὐτῷ τῶν μαθηματικῶν ὀργάνων σκιόθηρα καὶ σφαίρας καὶ γωνίας, αἷς ἐναρμόττει τὸ τοῦ ἡλίου μέγεθος πρὸς τήν ὄψιν, στρατιῶται περιτυχόντες καὶ χρυσίον ἐν τῷ τεύχει δόξαντες φέρειν ἀπέκτειναν. ὅτι μέντοι Μάρκελλος ἤλγησε καὶ τὸν αὐτόχειρα τοῦ ἀνδρὸς ἀπεστράφη καθάπερ ἐναγῆ, τοὺς δὲ οἰκείους ἀνευρὼν ἐτίμησεν, ὁμολογεῖται.
Valerius Maximus 8.7.ext.7 (tr. D.R. Shackleton Bailey):
I should say that Archimedes' diligence also bore fruit if it had not both given him life and taken it away. At the capture of Syracuse Marcellus had been aware that his victory had been held up much and long by Archimedes' machines. However, pleased with the man's exceptional skill, he gave out that his life was to be spared, putting almost as much glory in saving Archimedes as in crushing Syracuse. But as Archimedes was drawing diagrams with mind and eyes fixed on the ground, a soldier who had broken into the house in quest of loot with sword drawn over his head asked him who he was. Too much absorbed in tracking down his objective, Archimedes could not give his name but said, protecting the dust with his hands, "I beg you, don't disturb this," and was slaughtered as neglectful of the victor's command; with his blood he confused the lines of his art. So it fell out that he was first granted his life and then stripped of it by reason of the same pursuit.
Archimedis quoque fructuosam industriam fuisse dicerem, nisi eadem illi et dedisset vitam et abstulisset: captis enim Syracusis Marcellus <etsi> machinationibus eius multum ac diu victoriam suam inhibitam senserat, eximia tamen hominis prudentia delectatus ut capiti illius parceretur edixit, paene tantum gloriae in Archimede servato quantum in oppressis Syracusis reponens. at is, dum animo et oculis in terra defixis formas describit, militi, qui praedandi gratia <in> domum irruperat strictoque super caput gladio quisnam esset interrogabat, propter nimiam cupiditatem investigandi quod requirebat nomen suum indicare non potuit, sed protecto manibus pulvere 'noli' inquit, 'obsecro, istum disturbare,' ac perinde quasi neglegens imperii victoris obtruncatus sanguine suo artis suae liniamenta confudit. quo accidit ut propter idem studium modo donaretur vita, modo spoliaretur.
etsi add. Gertz
in add. Kempf
Silius Italicus 14.676-678 (tr. J.D. Duff):
too, O famous
man, defender of thy native
city, didst win tears
from the conqueror.
Archimedes was calmly poring
sand, when the great
disaster came down upon him.
tu quoque ductoris lacrimas, memorande, tulisti,
defensor patriae, meditantem in pulvere formas
nec turbatum animi tanta feriente ruina.
See E.J. Dijksterhuis, Archimedes
, tr. C. Dikshoorn (Princeton: Princeton University Press, 1987), pp. 30-32, and Mary Jaeger, Archimedes and the Roman Imagination
(Ann Arbor: University of Michigan Press, 2008), pp. 77-100.