Muawiya was traveling through Byzantine territory while a treaty was still in effect between him and them. He intended to draw close to them so that when the treaty expired he could launch an attack. Then an old man on a riding beast appeared, saying, "Allah is the greatest, Allah is the greatest. Faithfulness, not treachery. The Messenger of Allah ﷺ said, 'Whoever has a treaty between himself and a people must not loosen any knot of it nor tighten one until its term has expired, or else he must openly renounce it on equal terms.'"When this reached Muawiya, he turned back. The old man turned out to be Amr bin Abasa.
حدثنا محمد بن جعفر قال حدثنا شعبة عن أبي الفيض
كان معاوية يسير بأرض الروم وكان بينهم وبينه أمد فأراد أن يدنو منهم فإذا انقضى الأمد غزاهم فإذا شيخ على دابة يقول الله أكبر الله أكبر وفاء لا غدر إن رسول الله ﷺ قال من كان بينه وبين قوم عهد فلا يحلن عقدة ولا يشدها حتى ينقضي أمدها أو ينبذ إليهم على سواء فبلغ ذلك معاوية فرجع وإذا الشيخ عمرو بن عبسة
Its isnad is sahih. Its narrators are well known, and Abu Faid is Shami, mentioned many times before, whose name is Musa bin Ayub. Ibn Hibban and Ajli rated him trustworthy, and Abu Hatim considered him sound. Salim bin Amir is Kalai Himsi, the jurist, one of the senior trustworthy Followers, and he is said to have had a sighting of the Prophet.The hadith was narrated by Tirmidhi 4/143, no. 1580, in the chapters on military expeditions, on what has been said about treachery, and he said, hasan sahih. It was also narrated by Ibn Abu Shayba 12/459, no. 15255, in the chapter on jihad, on treachery in granting safe conduct, and by Bayhaqi 9/231.