An extreme Shia asked one of Ahly Sunna: "Who is better than four with whom the Messenger of Allah is the fifth?"
He was alluding to Ali, Fatima, Hasan and Hussain when the Prophet wrapped his mantle around them in the hadith of mubahala.
The Sunni replied: "Two with whom Allah Most High is the third."
He was alluding to the Prophet and Abu Bakr in the cave when the Prophet said to him: "What do you think of two of which Allah is the third?"
Narrated in Tabaqat Shafi`iyya al Kubra (Hijr ed. 5:98).