The Prophet (PBUH&HF) had said...
The Prophet (PBUH&HF) had said: "To love Ali (AS) is a sign of Faith, and to hate him (AS) is a sign of hypocrisy!!!" This tradition is narrated in Sahih Muslim, v1, p61. Check for yourself.
In Sahih al-Bukhari, v2, p76; and Sahih al-Tirmidhi, v5, p300 narrate that the Prophet (PBUH&HF) said to Ali (AS): "You are a part of me, and I a part of you." Also, Sahih al-Tirmidhi, v5, p201 narrates that the Prophet (PBUH&HF) said: "I am the city of knowledge, and Ali is it's door." Keep in mind that you can only enter a city through its door; meaning that any knowledge from the Prophet (PBUH&HF), since he (PBUH&HF) is the City of Knowledge, can only be accessed through the door, his (PBUH&HF) son-in-law Ali (AS).
What's more, Musnad al-Imam Ibn Hanbal, v5, p25, narrates that the Prophet (PBUH&HF) said: "Ali is the master of every believer after me." If ANY head of state, whether today or in times immemorial, ALWAYS has a trusted vicegerent to take his place and manage his affairs in his absence, would you then believe that the Prophet (PBUH&HF), who was sent as the FINAL Messenger from He (SWT) Who created the universe, didn't also have a vicegerent to manage his (PBUH&HF) affairs after his (PBUH&HF) death?
A vicegerent that Allah (SWT) also trusts and loves? Would you believe that Allah (SWT) would leave the affairs of the "...Best of Nations sent forth to mankind...[3:110]" to random selection and ruling? No, by Allah (SWT), a vicegerent was INDEED chosen by Allah (SWT) and His (SWT) Messenger (PBUH&HF), and that vicegerent was al-Imam Ali Ibn Abi Talib (AS). Again, Sahih Tirmidhi, v2, p298, narrates that the Prophet (PBUH&HF) said: "Whoever I was his Master, then Ali is his Master!
O Allah, support those who support him, and alienate those that alienate him!!!" That is Ali (AS), the fearless warrior, and the defeater of the champions of Quraish. May the Blessings and Peace of Allah (SWT) be extended to the Prophet (PBUH&HF) and his (PBUH&HF) Family, Amen. Now, ask yourself: If this is how the Prophet (PBUH&HF) praised Ali (AS), then who are the companions, especially Muawiyah, to curse him (AS)?