Musnad Ahmad Hadith 17025

Saleem bin Aamir says that once Ameer Muawiya (may Allah be pleased with him) set out with an army towards the land of Rome. The agreed period of the treaty between Muawiya (may Allah be pleased with him) and the Romans was still remaining. Muawiya (may Allah be pleased with him) thought that he would stop after reaching near them, and as soon as the period ended, he would start fighting them. But what he saw was that an old man riding a mount was going along saying, "Fulfill the promise, do not break the covenant." The Prophet (ﷺ) said: "If a person has a treaty with any nation, he should not untie or tie its knot before the period ends or before they break the covenant from their side." When Muawiya (may Allah be pleased with him) came to know of this, he turned back, and it was found out that the old man was Amr bin Abasah (may Allah be pleased with him).