It is narrated from the fifth Shia Imam, Mohammad Al-Baqir (AS) who said:

Islam is based on five elements of Salaat/Salaah, Zakat, fasting, Hajj, and Welayah; and nothing but Welayah has been so important and emphasised, because it is the most vital element. Then, the people got hold of four elements and left behind the Welayah [1].

[1] Al-Kafi, Sheikh Kolayni (RA), Vol. 2, Page 18, Bab Da’a’em-ul-Islam, Hadith 3.

Source: en.shafaqna.com