Musnad Ahmad 658:57

Isma'il narrated to us, Layth narrated to us, from 'Amr ibn Murrah, from Mu'awiyah ibn Suwayd ibn Muqarrin, from Al-Bara' ibn 'Azib, who said: We were sitting with the Prophet, peace and blessings of Allah be upon him, and he said: "Which of the bonds of Islam is the most secure?" They said: "Prayer." He said: "It is good, but it is not that." They said: "Zakat." He said: "It is good, but it is not that." They said: "Fasting Ramadan." He said: "It is good, but it is not that." They said: "Hajj." He said: "It is good, but it is not that." They said: "Jihad." He said: "It is good, but it is not that." He said: "The most secure bond of faith is that you love for the sake of Allah and hate for the sake of Allah."