Sulaim b. ‘Amir said

There was a covenant between Mu'awiya and the Byzantines, and he was going towards their country; then when the covenant came to an end he attacked them. A man came on a horse, or a packhorse, saying, “Allah is most great, Allah is most great; let there be faithfulness and not treachery.”* And when they looked they found that he was 'Amr b. ‘Abasa. Mu'awiya questioned him about that and he replied that he had heard Allah's Messenger ﷺ say, "When one has a covenant with people he must not loosen or strengthen it till its term comes, or he brings it to an end in agreement with them.” He said that Mu'awiya then took the people back. *Mirqat, 4, 255 says that 'Amr b. ‘Abasa felt it was an act of treachery to make a treaty while in one’s own country and then approach the enemy’s country when the period of the treaty was near an end, so as to be able to make an immediate attack. Tirmidhi and Abu Dawud transmitted it.