Hadith al-thaqalayn is a famous and mutawatir (frequent) hadith from the Prophet (s) which implies people should follow the Qur'an and Ahl al-Bayt (a) after the Holy prophet's (s) demise. Based on this hadith, Muslims should refer to the Qur'an and Ahl al-Bayt (a) and these two are inseparable.
Relying on the hadith of thaqalayn, Shi'a scholars prove the necessity of Imam, necessity of infallibility of Imams, and the necessity of continuation of Imamate in all times.

"Indeed I am leaving two things among you, to which if you hold yourself, you will never astray: the book of Allah –who is all mighty and great- and my ahl al-bayt (household), my 'itra (family). O people hear! And I have announced to you that: indeed you will enter my presence and I will ask you about what you did to the thaqalayn (two weighty things) and the thaqalayn are the book of Allah and my ahl al-bayt."