I (the narrator) asked Abu Abdullah (as) about the major sins, so he (as) said: These are seven in the Book of Ali (as): 1. Disbelief in Allah (swt), 2. Killing a soul (unjustly), 3. Disloyalty to parents, 4. Consuming the usury after the clarification, 5. Consuming the wealth of orphans unjustly, 6. Fleeing from the battle field, and 7. Adopting pre-islamic practices (the ones that were practiced before the hijrat).
I further asked: Is usurping a dirham of an orphan more grievous or the neglecting of salat? He (as) said: Neglecting the salat.
I said: But you (as) did not mention the neglecting of the salat as one of the major sins! So he (as) said: Tell me the first (item in the list) which I mentioned to you!
I said: The Kufr. He (as) said: The neglector of the salat, without a valid reason, is a kaafir.
[Al Kafi V 2 – The Book Of Belief and Disbelief CH 112 H 8]