Ali b. Aasim narrates from Muhammad b. Ali b. Moosa (a.s.) who, in turn, narrates from his father (a.s.) who relates from his ancestor Husain b. Ali (a.s.) who recounts, ‘I approached the Prophet of Allah (s.a.w.a.) while he was with Ubayy b. Kaab. The Holy Prophet (s.a.w.a.) turned towards me and declared – Allah has placed a pure, blessed, immaculate and undefiled clot in Hasan Askari’s (a.s.) groin. Only that believer will believe in him whom Allah has tested through his mastership. Allah will test the people through him, and the believer alone will endorse his Wilayat, while the disbeliever will renounce him. He is a pious, virtuous and an immaculate Imam for the people. He is a guide, as also the guided one. He shall issue verdicts based on the canons of justice and equity and shall command the people to act justly.’
Faraaed al-Simtain chap 35 trad 447