It was narrated from Warrad that: Mu'awiyah wrote to Al-Mughirah asking him to write him a hadith that he had heard from the Messenger of Allah (ﷺ). Al-Mughirah wrote to him (Saying): "I heard him say, when he finished the prayer: 'La Ilaha Illallah wahdahu la sharika lah, lahul-mulk wa lahul-hamd wa huwa 'ala kulli shay'in qadir (There is none worthy of worship except Allah (ﷺ) alone with no partner or associate. He is the Dominion and to Him be all praise, and He is able to do all things) three times."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا الْحَسَنُ بْنُ إِسْمَاعِيلُ الْمُجَالِدِيُّ، قَالَ أَنْبَأَنَا هُشَيْمٌ، قَالَ أَنْبَأَنَا الْمُغِيرَةُ، وَذَكَرَ، آخَرَ ح وَأَنْبَأَنَا يَعْقُوبُ بْنُ إِبْرَاهِيمَ، قَالَ حَدَّثَنَا هُشَيْمٌ، قَالَ أَنْبَأَنَا غَيْرُ، وَاحِدٍ، مِنْهُمُ الْمُغِيرَةُ عَنِ الشَّعْبِيِّ، عَنْ وَرَّادٍ، كَاتِبِ الْمُغِيرَةِ أَنَّ مُعَاوِيَةَ، كَتَبَ إِلَى الْمُغِيرَةِ أَنِ اكْتُبْ إِلَىَّ بِحَدِيثٍ سَمِعْتَهُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم . فَكَتَبَ إِلَيْهِ الْمُغِيرَةُ إِنِّي سَمِعْتُهُ يَقُولُ عِنْدَ انْصِرَافِهِ مِنَ الصَّلاَةِ " لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَىْءٍ قَدِيرٌ " . ثَلاَثَ مَرَّاتٍ .
sunnah.com