Hadith Al-Munasabah

The Holy Prophet (pbuh&hp) said:
“Whoever contests Ali in regard to the khilafah is an unbeliever.”
Sunni Refernces:

• Ibn al Maghazili in his Manaqib (Tehran), p.45, from Abu Dharr al Ghifari,
• Allamah Ayni Hyderabadi in Manaqib Sayyidina Ali (Alam Press, Charminar), p.52, from al Khatib
  al Khwarazmi and Ibn al Maghazili.