Quintus Curtius Rufus, History of Alexander 8.13.3

8.13.3

Iam Hydaspen Alexander superare decreverat, cum Barzaentes, defectionis Arachosiis auctor, vinctus trigintaque elephanti simul capti perducuntur, opportunum adversus Indos auxilium: quippe plus in beluis quam in exercitu spei ac virium illis erat.

On the following day envoys from Abisares reached the king, and, as they had been instructed, surrendered to him all that their master possessed. After pledges of good faith had been interchanged, they were sent back to their sovereign. Alexander, thinking that by the mere prestige of his name Porus also would be induced to surrender, sent Cleochares to tell him in peremptory terms that he must pay tribute and come to meet his sovereign at the very frontiers of his own dominions. Porus answered that he would comply with the second of these demands, and when Alexander entered his realm he would meet him, but come armed for battle. Alexander had now resolved to cross the Hydaspes, when Barzaentes, who had instigated the Arachosians to revolt, was brought to him in chains, along with thirty captured elephants, an opportune reinforcement against the Indians, since these huge beasts more than the soldiery constituted the hope and main strength of an Indian army.