One must believe that the Prophet, salla-l-lahu ^alayhi wa sallam, is truthful in everything he told about, and conveyed from Allah, whether it was about the news of previous nations and the beginning of the creation, or about the permissible or prohibited actions and sayings, or about what will happen in this life and in the Hereafter. Allah ta^ala said in Surat An-Najm verse #3-4:

﴿وما ينطق عن الهوى، إن هو إلا وحيٌ يوحى﴾ سورة النجم.

The verse means: [Nor does he speak from [his own] inclination. It is not but a revelation revealed.]