Thomas Heath, A History of Greek Mathematics. Volume I: From Thales to Euclid, Clarendon Press, Oxford, 19211

p. 24, Chapter I:

Plato’s attitude towards mathematics was, as we have seen, quite exceptional; and it was no doubt largely owing to his influence and his inspiration that mathematics and astronomy were so enormously advanced in his school, and especially by Eudoxus of Cnidos and Heraclides of Pontus. But the popular attitude towards Plato’s style of lecturing was not encouraging. There is a story of a lecture of his on ‘The Good’ which Aristotle was fond of telling. (Aristoxenus, Harmonica, ii ad init.) The lecture was attended by a great crowd, and ‘every one went there with the idea that he would be put in the way of getting one or other of the things in human life which are usually accounted good, such as Riches, Health, Strength, or, generally, any extraordinary gift of fortune. But when they found that Plato discoursed about mathematics, arithmetic, geometry, and astronomy, and finally declared the One to be the Good, no wonder they were altogether taken by surprise; insomuch that in the end some of the audience were inclined to scoff at the whole thing, while others objected to it altogether.’ Plato, however, was able to pick and choose his pupils, and he could therefore insist on compliance with the notice which he is said to have put over his porch, ‘Let no one unversed in geometry enter my doors’ (Tzetzes, Chiliad. viii. 972); and similarly Xenocrates, who, after Speusippus, succeeded to the headship of the school, could turn away an applicant for admission who knew no geometry with the words, ‘Go thy way, for thou hast not the means of getting a grip of philosophy’. (Diog. L., iv. 10)

The Harmonics of Aristoxenus, edited with translation, notes, introduction and index of words by Henry S. Macran, Clarendon Press, Oxford, 19022

p. 187, Book II, Sections 30-31:

It will be well perhaps to review in anticipation the course of our study; thus a foreknowledge of the road that we must travel will enable us to recognize each stage as we reach it, and so lighten the toil of the journey; nor shall we be harbouring unknown to ourselves a false conception of our subject. Such was the condition, as Aristotle used often to relate, of most of the audience that attended Plato’s lectures on the Good. They came, he used to say, every one of them, in the conviction that they would get from the lectures some one or other of the things that the world calls good; riches or health, or strength, in fine, some extraordinary gift of fortune. But when they found that Plato’s reasonings were of sciences and numbers, and geometry, and astronomy, and of good and unity as predicates of the finite, methinks their disenchantment was complete. The result was that some of them sneered at the thing, while others vilified it. Now to what was all this trouble due? To the fact that they had not waited to inform themselves of the nature of the subject, but after the manner of the sect of word-catchers had flocked round open-mouthed, attracted by the mere title ‘good’ in itself.

John Tzetzes, Chiliades, Book 8. Translated by Vasiliki Dogani, Theoi Project3

8.94 Concerning the inscription at the front door of Plato’s house “Let no one ignorant of geometry enter” (Story 249)
Plato had written at the front door of his house:
“Let no one ignorant of geometry enter my house.”
That is, let no one who is unjust come in here;
For geometry is fairness and justice.

Diogenes Laërtius, Lives of the Eminent Philosophers. Translated by R. D. Hicks, Loeb Classical Library4 5

Book IV, Chapter 2 (Xenocrates), Section 10:

When a little sparrow was pursued by a hawk and rushed into his bosom, he stroked it and let it go, declaring that a suppliant must not be betrayed. When bantered by Bion, he said he would make no reply. For neither, said he, does tragedy deign to answer the banter of comedy. To some one who had never learnt either music or geometry or astronomy, but nevertheless wished to attend his lectures, Xenocrates said, “Go your ways, for you offer philosophy nothing to lay hold of.” Others report him as saying, “It is not to me that you come for the carding of a fleece.”