Is is authentic that prophet SAWW said no one loves Ali a.s. unless believer and no one hates him unless hypocrite?
If yes then what is the reason of this? and is this said about any other Sahaba or only about Ali a.s.?
Yes, it is accepted among Muslims that the Prophet (PBUH) had said that, and he did so only about Ali (AS). Ali (AS) was the first man to become a Muslim, even before Abu Bakr, and this is a proven fact in both Sunni and Shii sources.
-Sahih Muslim: Hadith 113
-Musnad Ahmed: Hadith 25523
-Sunan Al-Tirmithe: Hadith 3651