Macrobius, Saturnalia 5.19
“In the fourth book of the Aeneid, describing the death of Dido, Vergil says that a lock of her hair was cut away in these verses:
Proserpina had not yet cut away the tawny lock from her and condemned her head to Stygian Orcus.
Then Iris, sent by Juno, cut away Dido’s hair and brought it to Orcus. Vergil has not contrived this story out of nothing, as that otherwise very learned man Cornutus supposes when he adds this commentary to the verse:
Whence came this story that hair must be taken away from the dying is unknown; but Vergil is accustomed to invent things now and then in the old poetic fashion, as he did with the golden bough.
Thus writes Cornutus. But I am ashamed that such a great man, though steeped in Greek literature, did not know that most remarkable play of Euripides, the Alcestis. For in that play, Orcus is brought onto the scene bearing a sword to cut the hair of Alcestis, and he speaks thus:
This woman then will go to the home of Hades.
I proceed to her, so that I may begin the rite with my sword.
For one is sacred to the god below ground
Once this blade has consecrated the hair of their head.
So it is clear, I think, whom Vergil followed in introducing the part about cutting the hair. But the Greeks mean by ἁγνίσαι to consecrate to the gods, whence your poet says in the character of Iris,
Ordered by Juno, I bear this sacred hair to Dis, and loose you from your body.
In libro quarto in describenda Elissae morte ait quod ei crinis abscisus esset his versibus:
Nondum illi flavum Proserpina vertice crinem
Abstulerat, Stygioque caput damnaverat Orco.
Deinde Iris a Iunone missa abscidit ei crinem et ad Orcum refert. Hanc Virgilius non de nihilo fabulam fingit, sicut vir alias doctissimus Cornutus existimat, qui annotationem eiusmodi adposuit his versibus: Unde haec historia, ut crinis auferendus sit morientibus, ignoratur: sed adsuevit poetico more aliqua fingere, ut de aureo ramo. Haec Cornutus. Sed me pudet quod tantus vir, Graecarum etiam doctissimus litterarum, ignoravit Euripidis nobilissimam fabulam Alcestim. In hac enim fabula in scenam Orcus inducitur gladium gestans quo crinem abscidat Alcestidis, et sic loquitur,
ἡ δ᾽ οὖν γυνὴ κάτεισιν εἰς Ἅιδου δόμους.
στείχω δ᾽ ἐπ᾽ αὐτὴν ὡς κατάρξωμαι ξίφει:
ἱερὸς γὰρ οὗτος τῶν κατὰ χθονὸς θεῶν
ὅτου τόδ᾽ ἔγχος κρατὸς ἁγνίσῃ τρίχα.
Proditum est, ut opinor, quem secutus Virgilius fabulam abscidendi crinis induxerit: ἁγνίσαι autem Graece dicunt dis consecrare, unde poeta vester ait ex Iridis persona:
— Hunc ego Diti
Sacrum iussa fero, teque isto corpore solvo.