Saeed bin Asi reported,
He used to relate that Muawiya took over the water vessel after Abu Hurairah, following the Messenger of Allah ﷺ with it when Abu Hurairah fell ill. While he was pouring water for the Messenger of Allah ﷺ to perform ablution, the Prophet raised his head toward him once or twice and said, "Muawiya, if you are ever placed in charge of an affair, fear Allah 
and rule with justice."Muawiya said, "I never stopped thinking that I would be tested with authority because of the Prophet's ﷺ words, until I was indeed tested with it."
Ahmad 16933
Sahihقال حدثنا روح قال حدثنا
أبو أمية عمرو بن يحيى بن سعيد قال سمعت جدي
يحدث أن معاوية أخذ الإداوة بعد أبي هريرة يتبع رسول الله ﷺ بها واشتكى أبو هريرة فبينا هو يوضئ رسول الله ﷺ رفع رأسه إليه مرة أو مرتين فقال يا معاوية إن وليت أمرا فاتق الله عز وجل واعدل قال فما زلت أظن أني مبتلى بعمل لقول النبي ﷺ حتى ابتليت
No references are available.
Its isnad is sahih. Amr bin Yahya bin Saeed bin Amr bin Saeed bin As Umawi is trustworthy, as is his father Ashdaq, and his grandfather is among the trustworthy Successors, and here he narrates from his grandfather.The hadith is found in Abu Yaala 13/370, no. 7380, and in Bayhaqi's Dalail 6/446. Haythami said in 5/186 and 9/355-356 that it is mursal but its narrators are the narrators of the sahih works, and that Abu Yaala connected it, narrating from Saeed from Muawiya.I see no difference here, for Ahmad also narrates it from Saeed from Muawiya, since he says, "I heard my grandfather." So where is the mursal transmission or the break in the chain?