the imams' infallibility

Among the people of the Imamate, a branch of the Shi'a, the imam must be free from sin (ma'sum). This is the presentation of a particular sect's view, transmitted by al-Masudi, and not the position of the people of the Sunna. ¹

Al-Masudi says that they hold that if the imam were not free from sin, one could not be sure that he would not do what others do, and so he would need another imam, and so on without end. They hold that he is the most knowledgeable, bravest, and most generous of people, and that the imamate is an explicit designation by God and His Messenger of the specific imam, and that in every age there is a proof of God, outward or inward. ¹

The view here comes from a single non-Shi'i source. It needs a further source from the Imami books themselves, and then the academic view.