Cicero, Against Verres
II 1.20.54 (tr. L.H.G. Greenwood, with his note):
My charge is that Verres did not leave one single statue behind; that from temples and public places alike, with the whole of Aspendus looking on, they were all openly loaded on wagons and carted away. Yes, even the famous Harper of Aspendus, about whom you have often heard the saying that is proverbial among the Greeks, of whom it was said that he made "all his music inside"a—him too Verres carried off and put right inside his own house...
a The proverb was applied to those who do things for their own pleasure and not that of others. The lifelike figure appeared to be enjoying his own music, inaudible to everyone else.
hoc dico, nullum te Aspendi signum, Verres, reliquisse, omnia ex fanis, ex locis publicis, palam, spectantibus omnibus, plaustris evecta exportataque esse. atque etiam illum Aspendium citharistam, de quo saepe audistis id quod est Graecis hominibus in proverbio, quem omnia intus canere dicebant, sustulit et in intimis suis aedibus posuit...
George Long ad loc.:
Cicero has this expression (In Rullum, ii.26), "atque hoc carmen hic tribunus plebis non vobis, sed sibi intus canit." The explanation of this passage by Asconius, which Klotz has adopted, is the following:—"There was something in the manner in which this figure was represented as holding the lyre, to which the words 'intus canere' refer. Asconius says that the player on the cithara holds the 'plectrum' in his right hand, whiich is 'foris canere;' and he has the fingers of the left hand on the strings, which is 'intus canere.' This 'citharista' of Aspendus was represented doing every thing with the left band, 'intus;' he did not use the right hand at all. The proverbial expression 'intus canere' was also applied to a person who slily looked after his own interest." Asconius adds: 'unde omnes, quotquot fures erant, a Graecis Aspendii citharistae in proverbio dicebantur, quod, ut ille carminis, ita isti furtorum occultatores erant.' But this explanation of Asconius certainly does not explain the proverb. Zumpt gives a different sense
to the words 'intus canere,' as applied to the 'citharista:' "the statue was made with such skill, that the 'citharista' seemed to be feeling his music, or 'intus canebat;'" but nobody else, of course, could hear it. He had, then, all his playing to himself.