What is the ruling on someone who says in the Tashahhud, "Peace be upon you and upon the righteous servants of Allah," instead of, "Peace be upon us and upon the righteous servants of Allah"? Does this invalidate his prayer? And must he repeat it, knowing that he did not intend to make a mistake and has been in this state for years?

Reciting the final Tashahhud is a pillar (rukn) of prayer according to the Shafi'i and Hanbali schools of thought. The Tashahhud is sufficient in any of the formulations authentically reported from the Prophet, peace be upon him, with the best of them being the Tashahhud of Abdullah ibn Mas'ud. Whoever omits a single letter from the Tashahhud, their prayer is invalid according to a group of scholars. However, the Maliki and Hanafi schools say that the prayer of one who omits some words of the Tashahhud is valid, and some scholars do not deem it necessary for one to repeat a prayer invalidated due to ignorance.