It is quite well attested anciently.
There is Book 9 Chapter 12 of Pausanias' Description of Greece (which you have already noted in your own Answer). Roughly contemporaneous with Pausanias is Apollodorus' Bibliotheka 3.4.1:
When Telephassa died, Cadmus buried her, and after being
hospitably received by the Thracians he came to Delphi to inquire
about Europa. The god told him not to trouble about Europa, but to be
guided by a cow, and to found a city wherever she should fall down for
weariness. After receiving such an oracle he journeyed through Phocis;
then falling in with a cow among the herds of Pelagon, he followed it
behind. And after traversing Boeotia, it sank down where is now the
city of Thebes. Wishing to sacrifice the cow to Athena, he sent some
of his companions to draw water from the spring of Ares. But a dragon,
which some said was the offspring of Ares, guarded the spring and
destroyed most of those that were sent. In his indignation Cadmus
killed the dragon, and by the advice of Athena sowed its teeth. When
they were sown there rose from the ground armed men whom they called
Sparti. These slew each other, some in a chance brawl, and some in
ignorance. But Pherecydes says that when Cadmus saw armed men growing
up out of the ground, he flung stones at them, and they, supposing
that they were being pelted by each other, came to blows. However,
five of them survived, Echion, Udaeus, Chthonius, Hyperenor, and
Pelorus.
That is from James George Frazer's 1921 translation of the text, one of whose footnotes on this passage also points out that the visit is mentioned in the scholion on Homer's Iliad 2.494 as well as the one on Aeschylus' Seven Against Thebes 486. This footnote also says that "The Scholiast on Homer (l.c.) agrees almost verbally with Apollodorus, and cites as his authorities the Boeotica of Hellanicus and the third book of Apollodorus. Hence we may suppose that in this narrative Apollodorus followed Hellanicus" [emphasis added].
Fast-rewinding to about a century and a half prior to Pausanias and the Bibliotheka, the Roman writers Hyginus and Ovid each make mention of this story. Hyginus' Fabulae 178 (in Mary Grant's 1960 translation) says:
Europa was the daughter of Argiope and Agenor, a Sidonian.
Jupiter, changing his form to that of a bull, carried her from Sidon
to Crete, and begat by her Minos, Sarpedon, and Rhadamanthus. Her
father Agenor sent his sons to bring their sister back, or else not to
return to his sight. Phoenix set out for Africa, and there remained.
From this the Africans are called Phoenicians. Cilix from his own name
gave the name to Cilicia. Cadmus in his wanderings came to Delphi.
There the oracle told him to buy from farmers an ox which had a
moon-shaped mark on its side, and to drive it before him. Where it lay
down it was fated that he found a town and rule. When Cadmus heard the
oracle, he did as he was told. While seeking water he came to the
fountain of Castalia, which a dragon, the offspring of Mars, was
guarding. It killed the comrades of Cadmus, but was killed by Cadmus
with a stone. Under Minerva’s instructions he sowed the teeth and
ploughed them under. From them sprang the Sparti. These fought
themselves, but from them five survived, namely, Chthonius, Udaeus,
Hyperenor, Pelorus, and Echion. Moreover, Boeotia was named from the
ox Cadmus followed.
In Book 3 of the Metamorphoses, Ovid has his own narration of the story, although therein he never mentions the word Delphi. He refers to Phoebique oracula, "the oracle of Phoebus" (A.S. Kline's translation, as below, interprets it a step further, having "Apollo's oracle"), in Line 8, and "Castalia's Grotto" in Line 14.
And now the god, dispensing with the deceptive image of the bull,
confessed who he was, and made for the fields of Crete. Meanwhile
Europa’s father, in ignorance of this, orders his son Cadmus to search
for the stolen girl, and adds that exile is his punishment if he fails
to find her, showing himself, by the same action, both pious and
impious. Roaming the world (for who can discover whatever Jupiter has
taken?) Agenor’s son, the fugitive, shuns his native land and his
parent’s anger and as a suppliant consults Apollo’s oracle and asks in
what land he might settle. Phoebus replies ‘A heifer will find you in
the fields, that has never submitted to the yoke and is unaccustomed
to the curved plough. Go where she leads, and where she finds rest on
the grass build the walls of Thebes, your city, and call the land
Boeotia.’
Cadmus had scarcely left the Castalian cave when he saw an
unguarded heifer, moving slowly, and showing no mark of the yoke on
her neck. He follows close behind and chooses his steps by the traces
of her course, and silently thanks Phoebus, his guide to the way. Now
he had passed the fords of Cephisus and the fields of Panope: the
heifer stopped, and lifting her beautiful head with its noble horns to
the sky stirred the air with her lowing. Then looking back, to see her
companion following, she sank her hindquarters on the ground and
lowered her body onto the tender grass. Cadmus gave thanks, pressing
his lips to the foreign soil and welcoming the unknown hills and
fields.
Intending to offer a sacrifice to Jupiter, he ordered his
attendants to go in search of water from a running stream for a
libation. There was an ancient wood there, free from desecration, and,
in the centre of it, a chasm thick with bushes and willow branches,
framed in effect by stones making a low arch, and rich with copious
springs. There was a snake sacred to Mars concealed in this cave, with
a prominent golden crest. Fire flickered in its eyes, its whole body
was swollen with venom, its three-forked tongue flickered, and its
teeth were set in a triple row.
After the people of Tyre, setting out, a fatal step, reached the
grove, and let their pitchers down into the water, it gave out a
reverberation. The dark green snake thrust his head out of the deep
cavern, hissing awesomely. The pitchers fell from their hands, the
blood left their bodies, and, terrified, a sudden tremor took
possession of their limbs. The snake winds his scaly coils in restless
writhings, and, shooting upwards, curves into a huge arc. With half
its length raised into thin air, it peers down over the whole wood,
its body as great, seen in its entirety, as that Dragon that separates
the twin constellations of the Bear. Without pause he takes the
Phoenicians, whether they prepare to fight, run, or are held by fear
itself. Some he slays with his bite, some he kills in his deep
embraces, others with the corrupting putrefaction of his venomous
breath.
[3.1-49]
According to William Smith's 1888 Dictionary of Greek and Roman Biography and Mythology, the scholion on Aristophanes' Frogs 1256 mentions the story, as does the one on Euripides' Phoinissai 638. Frazer mentions the latter of these two scholia in the above-mentioned footnote as well, saying that this scholion "quotes the oracle at full length".
In Early Greek Mythography Vol. 2: Commentary, Robert Louis Fowler cites the 300s BC Athenian comic poet Eubulus, whose work currently survives only in fragments, as having said, in Fragment 33 (entitled Europa), that the Pythia instructed Cadmus to "found a city of Boiotoi [Boeotians]".
