who are the Shia?

بسم الله الواحد القهار

Sheikh As-Saduq:

The Shia were created from the same clay of the Imams:

My father, Allah may have mercy upon him, narrated to us from Abdullah Ibn Ja’far from Ahmad Ibn Mohammad from Ibn Abi Najran that he heard Abu al-Hasan (Imam ar-Ridha) saying:
He who antagonizes our Shia is antagonizing us and whoever follows them is following us, because they are part of us. They were created from the same clay from which we were created. He who loves them is one of us, and whoever hates them is not with us. Our Shia see through the light of Allah, move about in the mercy of Allah, and will win the dignity of Allah. We feel the same pains that any of our Shia feels during ailment, we become grieved whenever any of our Shia becomes grieved, and we feel happy whenever any of our Shia feels happy. No one of our Shia can be out of our auspices, whether he was in the east or the west of the earth. The debt that is left by any of our Shia is in our responsibility while the estate is his heirs’. Our Shia are those who offer the prayers, defray the zakat, perform the hajj to the Holy House of Allah, fast during the month of Ramadan, declare their loyalty to the Prophet’s household, and declare their disavowal of the enemies of the Prophet’s household. 
Those are the people of faith, God-fearing, piety, and devoutness. He whoever rejects them is rejecting Allah, because they are the true servants and the genuine disciples of Allah. By Allah I swear, each individual of them will be granted (on the Day of Resurrection) the right to intercede for people as many as the individuals of the tribes of Rabeea and Madhar, out of Allah’s honor to them.

Grading: Sahih(authentic)

Check the narrators of this Hadith in the Rijaal section:

1231: Ali Ibn Al-Husain Ibn Musa Ibn Babawaih(father of Sheikh As-Saduq)
1050: Abdullah Ibn Ja'far Al-Himyari
130: Ahmad Ibn Mohammad Ibn Isa Ibn Abdullah Al-Ash'ari Al-Qummi
992: Abdur-Rahman Ibn Abi Najraan

Source: Sifaat Ash-Shia, Pg. H-No. 3