سنن أبي داود

Narrated Amr ibn Abasah:

Sulaym ibn Amir, a man of Himyar, said: There was a covenant between Mu'awiyah and the Byzantines, and he was going towards their country, and 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 ibn Abasah. Mu'awiyah sent for him and questioned him (about that). He said: I heard the Messenger of Allah (ﷺ) say: When one has covenant with people he must not strengthen or loosen it till its term comes to an end or he brings it to an end in agreement with them (to make both the parties equal). So Mu'awiyah returned.