Quintus Curtius Rufus, History of Alexander 8.13.2

8.13.2

Porum quoque nominis sui fama ratus ad deditionem posse conpelli misit ad eum Cleocharen, qui denuntiaret ei, ut Stipendium penderet et in primo suorum finium aditu occurreret regi. Porus alterum ex his facturum sese respondit, ut intranti regnum suum praesto esset, sed armatus.

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.