Imam Ali bin Musa al-Rida is reported to have said:

Faith is greater than [the acceptance of] Islam by one degree, piety is greater than faith by one degree, and certainty is greater than piety by one degree. The children of Adam were not bestowed with something greater than certainty.

Shaykh al-Majlisi, Bihar al-anwar, vol 75, p. 338

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Source: imam-us.org