Numerous hadiths have been narrated from the Prophet (PBUH) indicating that love for Imam Ali (a.s) is a sign of having faith.
رسولُ اللَّهِ صلى اللَّه عليه وآله : حُبُّ عليٍّ يأكُلُ الذُّنوبَ كما تأكُلُ النّارُ الحَطَب
Translation: The Prophet (SAWA) said, 'Love for Ali consumes sins as fire consumes wood.[1]
رسولُ اللَّهِ صلى اللَّه عليه وآله : عُنوانُ صحيفةِ المؤمنِ حُبُّ عليِّ بنِ أبي طالبٍ
Translation: The Prophet (SAWA) said, 'The title of the deeds of a believer is the love for Ali b. Abu Talib.[2]
رسولُ اللَّهِ صلى اللَّه عليه وآله : ما ثَبّتَ اللَّهُ حُبَّ عليٍّ في قلبِ مؤمنٍ فَزلَّتْ به قدمٌ إلّا ثَبّتَ اللَّهُ قَدَماً يومَ القيامةِ علَى الصِّراط
Translation: The Prophet (SAWA) said, 'When Allah secures the love for Ali in a believer's heart, whenever his foot slips, Allah will secure his foot on the Sirat [Bridge outstretched over Hell] on the Day of Resurrection.[3]
رسولُ اللَّهِ صلى اللَّه عليه وآله - لِعَليٍّ عليه السلام - : لا يُحِبُّكَ إلّا مؤمنٌ ، ولا يُبغِضُكَ إلّا منافق
Translation: The Prophet (SAWA) said to Ali, 'None will love you but a believer and none will hate you but a hypocrite.' [4][5]
Footnotes:
[1] Kanz al-Ummal, no. ۳۳۰۲۱.
[2] Ibid. no. ۳۲۹۰۰
[3] Ibid. no. ۳۳۰۲۲
[4] Ibid. no. ۳۲۸۷۸
[5] http://www.hadith.net/en/post/32149/imam-ali