We Read in Alkafi, Vol 2, P18-19:
Imam Al-Baqir(as) says:
Islam is built upon five things: upon salat, zakat, hajj, sawm (fasting), and wilayah.
Zurarah said: I asked: Which of these is most excellent?
He said: Wilayah is most excellent because it is the key to them, and the wali is the guide to them.
I said:Then what follows it in excellence?
He said: Salat.
I said:Then what follows it in excellence?
He said: Zakat, because He (Allah) paired it with salat, and mentioned with salat before it.
I said: Then what follows it in excellence?
He said:Hajj.
I said: What follows it?
He said: Sawm (fasting).