In the same way as Allah has appointed heart as the leader of all organs, Hazrat Mahdi (as) will be theleader of the believers in the End Times

Abu Abdullah (Imam Ja’far al-Sadiq [a.s]) reports: ""O Hesham, would you tell us about your conversation with Amribn Obeid and the question you asked him?

Upon this request Hesham explained what he talked with Amribn Obeid and said: “Did Allah create the heart so that it could overcome the doubts of other organs in the body? (Amribn Obeid) said: ‘Yes’. Hesham said: the Almighty Creator did not leave the organs of your body without any guide and He appointed them a leader to make the truth evident and bring forth the absolute conviction regarding doubtful matters. Would He Who has done this leave humans in perplexity, confusion and controversy? Would He not appoint them a leader for them to refer to when He assigns a leader for your organs so that they are free of their doubts and confusions?

Abu Abdullah (Imam Ja’far al-Sadiq [a.s]) smiled and said: ‘O Hesham! Who taught you these? I said: “I learned these from you and brought them together.’ Imam said: ‘I swear by Allah that these are the truths  in the pages of Abraham and Moses.” (Usul al-Kafi, YakubibnIshaq al-Kulayni, vol.1, page 229)