The main source is Livy's History of Rome since the Foundation 8.17.8-10 (referring to the year 332 VC = 329/328 BCE) and 8.24 (referring to 326 VC = 323/322 BCE, but in fact 331 BCE). The two sections are offered here in the translation by B.O. Foster.
[8.17.8-10] Samnium likewise had now for two years been suspected of hatching revolutionary schemes, for which reason the Roman army was not withdrawn from the Sidicine country. But an invasion by Alexander of Epirus drew the Samnites off into Lucania, and these two peoples engaged in a pitched battle with the king, as he was marching up from Paestum. The victory remained with Alexander, who then made a treaty of peace with the Romans; with what faith he intended to keep it, had the rest of his campaign been equally successful, is a question.