‘Abdullah b. Mas’ud said: The Prophet صلى الله عليه وسلم said to me: O ‘Abdallah b. Mas’ud! I said three times: “Here I am O Messenger of Allah.” He said: “Do you know which bond of Imaan is the strongest?” I said: “Allah and His Messenger know best.” He said: “The strongest Imaan is the loyalty for Allah’s sake, by loving and hating for Allah’s sake…”

Narrated by al-Hakim in his al-Mustadrak