Usul al-Kafi
Book 4:
The Book of Divine Proof

Chapter 96
The duties of believers after Hajj

491. 2. Sadeer reports that when he was leaving the sacred premises of the Holy Mosque of Kaba, Imam Muhammad al-Baqir (p) entered the Mosque. The Imam (p) held his hand and said: People have been ordered to circumambulate these premises and then come to us to express their love and devotion to us.
Then the Imam (p) quoted the verse: And verily, I am most forgiving towards him who repents and believes and acts righteously and follows the guidance... (Q, 20:82), and then he pointed towards his chest and said: They should have full faith in our Imamate.