Although the majority of Muslims believe in the 'ismah of the prophets, there is a great difference about the extent of their 'ismah.
As far as the Sunnis are concerned, they have a great difference of opinion among themselves. Their views are as the followings:
On lying & infidelity (kufr): all Sunnis believe that prophets could not tell a lie, neither intentionally nor by mistake, nor could they be infidel before or after the declaration of their prophethood.
Other sins intentionally: all Sunnis believe that the prophets could not commit other sins intentionally.
Major sins unintentionally: majority believes that the prophets could commit such sins; however, a minority says that this is not possible.
Minor sins: majority believes that the prophets could commit minor sins, though not such minor sins which would disgrace them in public's eyes.1
The Shí'as Muslims believe that all the Prophets were ma'sum, sinless and infallible; they could commit no sin— neither a major sin nor a minor sin; neither intentionally nor inadvertently; and this applies to them from the beginning to the end of their lives.
This is the belief of the Shí'a Ithna-'Ashariyyah. Below are three quotations from the Shí'a scholars of early days to the present century that shows the consistency of this belief among the Shí'as.
Shaykh Abu Ja'far as-Saduq, a scholar born during the Minor Occultation (ghaybat sughra) of the Present Imam and died in 381 AH, says: “Our belief concerning the prophets, apostles, Imams and angels is that they are infallible (ma'sum), purified from all defilement (danas), and that they do not commit any sin, whether it be minor or major…He who denies infallibility to them in any matter appertaining to their status is ignorant of them. Our belief concerning them is that they are infallible and possess the attributes of perfection, completeness and knowledge, from the beginning to the end of their careers…”2
'Allamah Ibn Mutahhar al-Hilli (d. 728 AH) writes the following on prophecy: “He is immune to sin from the first of his life to the last of it.”3
Shaykh Muhammad Rida al-Mudhaffar, a famous Shí'a scholar of the first half of this century, writes: “We believe that all the prophets are infallible…Infallibility means purity from all sins, both major and minor ones, and from mistakes and forgetfulness.”4