The History of the Grecian War (1.102.2-1.102.4)

They were sent for principally for their reputation in mural assaults, the long continuance of the siege seeming to require men of ability in that kind, whereby they might perhaps have gotten the place by force.

And upon this journey grew the first manifest dissension between the Lacedaemonians and the Athenians. For the Lacedaemonians, when they could not take the place by assault, fearing lest the audacious and innovating humour of the Athenians, whom withal they esteemed of a contrary race, might, at the persuasion of those in Ithome, cause some alteration if they stayed, dismissed them alone of all the confederates, not discovering their jealousy but alleging that they had no farther need of their service.

But the Athenians, perceiving that they were not sent away upon good cause but only as men suspected, made it a heinous matter, and conceiving that they had better deserved at the Lacedaemonians' hands, as soon as they were gone left the league which they had made with the Lacedaemonians against the Persian and became confederates with their enemies the Argives; and then both Argives and Athenians took the same oath and made the same league with the Thessalians.