Usul al-Kafi
Book 5:
The Book of Belief and Disbelief

Chapter 19
Excellence in belief

642. 1. Imam Jafar as-Sadiq (p) was asked: Are there any grades in belief in the eye of God due to which one believer is superior to another?
The Imam (p): Yes.
He was requested to explain.
The Imam (p): The superiority of one believer over another can be likened to a race of horses. Those who are forward in the lead are superior to the ones who lag behind. God has determined the status or superiority of Muslims according to their valour in Jihad (to strive in the way of God). God does not curtail anyone’s rights and does not give preference to an inferior over a superior. He has given preference to those fighters who go forward courageously on the battlefield…(B5,C19,T1)