Muawiya wrote to Mughira asking him to send him a hadith he had heard from the Messenger of Allah ﷺ. So Mughira wrote back to him saying that he had heard him say upon finishing his prayer, three times, "There is no god but Allah alone, with no partner. To Him belongs the dominion and to Him belongs all praise, and He is capable of all things."
أخبرنا الحسن بن إسمعيل المجالدي قال أنبأنا هشيم قال أنبأنا المغيرة وذكر آخر ح وأنبأنا يعقوب بن إبراهيم قال حدثنا هشيم قال أنبأنا غير واحد منهم المغيرة عن الشعبي
أن معاوية كتب إلى المغيرة أن اكتب إلي بحديث سمعته من رسول الله ﷺ فكتب إليه المغيرة إني سمعته يقول عند انصرافه من الصلاة لا إله إلا الله وحده لا شريك له له الملك وله الحمد وهو على كل شيء قدير ثلاث مرات
[Its chain is daif] It is found in Kubra, hadith no. 1266. Mughira bin Miqsam was a mudallis, as Nasai stated (Siyar Alam al-Nubala*, 747, and elsewhere), and I did not find explicit confirmation of his direct hearing. The basic hadith is agreed upon, Bukhari, hadith no. 844, and Muslim, hadith no. 593, without the addition of "three times," which is the preserved wording.