Ayesha in Yanabi al Mawaddah page 290:
Ata narrates: ‘I asked Ayesha about Ali. She replied: ‘He is the best of men; no one denies this except a Kaafir’’.
The same text has also been narrated from Hudhayfa, Jabir ibn Abdullah and Imam Ali (s) himself on the same page. Other Sunni scholars have also recorded this from the blessed lips of our Prophet (s); here is a list of where it can be located:
Tareekh Damishq Volume 2 page 942, by Ibn Asakir
Tarikh, by al-Khateeb al-Baghdadi, v3, page 193
Min Hadith Khaithema bin Sulaiman page by Khaithema bin Sulaiman Al-Qusashi
Kunuz Al-Haqa’iq, by Abdul Raouf al-Manawi, Volume 2 page 15, the letter Ayn
Faraid al Simtayn, Volume 1 page 154 Chapter 31
Kifayath al Muttalib Volume 9 page 245
Muwaddatul Qurba page 30
Kanz ul Ummal, Hadith 33046