Ahmad bin Shu'aib Al-Nasa'i (Died 202 AH) narrated like this :
أخبرنا محمد بن المثنى، قال: حدثنا أبو بكر الحنفي، قال: حدثنا بكير بن مسمار، قال: سمعت عامر بن سعد، يقول: قال معاوية لسعد بن أبي وقاص: ما منعك أن تسب علي بن أبي طالب؟» قال: لا أسبه ما ذكرت ثلاثاً .....
'Aamir bin Sa'ad narrated that Mu'awiyah told Sa'ad Ibn Abi Waqas : What prevents you from abusing abusing Ali (as). He replied : I will never abuse him after i heard about him three things.....
Sunan - Nasa'i // Volume 10 // Page 372 // Number 8584 // Edn.Dar Al-Tasil Riyadh Saudi Arabia.
Al-Nasa'i narrated same narration with same wording in his another book.
Khasais Ali Ibn Abi Talib - Nasa'i // Page 56 // Number 54 // Edn. Maktaba Asariyyah Beriut Lebanon.
Now the same narration has been narrated by Al-Bazzar (Died 292 AH) with same chain but he tampered with the text by changing the name of Mu'waiyah to Man (رجل).