According to the Chiose anonime alla prima cantica della Divina Commedia di un contemporaneo del poeta [Anonymous comments on the first cantica of the Divine Comedy by a contemporary of the poet], published in 1865 by Francesco Selmi, the source is a Laurentian codex [that is, a manuscript in the Laurentian Library in Florence]:
LAUR. XLII, 14: «Fu uomo superbissimo ed iracondo, al punto che una volta, avendo questione con Dante, diede uno schiaffo a Dante perchè erano di diverse e contrarie parti. E sempre fu inimicizia massima fra loro due, e perciò non meravigliare se il poeta dica male di lui ed anche della sua famiglia, come poscia nel canto XVI del Paradiso: la oltracotata gente (sic) che s'indraca».
My translation:
LAUR. XLII, 14: He [Filippo Argenti] was an extremely proud and hot-tempered man, to the point that, once, having a conflict with Dante, being on opposing and rival political factions, he slapped Dante. There was always great enmity between the two of them, and for this reason, one should not be surprised that the poet speaks ill of him and of his family as well, as he later does in Canto XVI of Paradiso: the haughty lineage [sic—the word is schiatta, not gente] that endragons itself.
There are several Laurentian manuscripts that contain the Divine Comedy, so I don't know which one this text is referring to. Also A. Linaker, in an article titled "Il canto dell'ira" [the canto of wrath], which appeared in 1907 in the journal Il Giornale Dantesco, writes:
È detto pure in un codice Laurenziano uomo superbissimo e iracondo al punto che una volta, avendo questione con Dante perché eran di diverse e contrarie parti, gli dètte uno schiaffo.
My translation:
It is also said in a Laurentian codex that he was an extremely proud and hot-tempered man, to the point that, once, having a conflict with Dante because they belonged to opposing and rival political factions, he slapped him.
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