Arrian, Anabasis 5.15.5-7
5 When he (Porus) found a place that he did not think muddy - it was sandy and therefore all level and solid for cavalry to charge and retire - he drew up his army there, putting the elephants on the front line, not less than a hundred feet apart, so as to form a line in front of the whole phalanx of infantry, and terrify the cavalry of Alexander at all points. 6 He did not in any case expect that any of the enemy would dare force way through the gaps between the elephants on horseback, since the horses would take fright; still less did he fear the foot soldiers, who would be kept back by the heavy armed troops advancing frontally and trampled down by the elephants turning upon them. 7 Next the elephants he stationed his foot soldiers, not on the same front as the beasts, but in a second line behind them, so that the companies were fitted more or less into the intervals. He also had foot soldiers stationed on the wings stretching even beyond the line of elephants; on each flank of the infantry the cavalry were posted, and in the front of them the chariots on both flanks.