Chapter on the Detailed Explanation of Prayer, Including the Obligatory and Recommended, and What is Permissible and Not Permissible.

As for what was narrated by al-Husayn ibn Sa’id from Safwan from Mansur from Abu Basir: Abu Abdillah عليه السلام said: "If a man is certain that he left out a rak’ah from the prayer, and he had already performed two prostrations but omitted the bowing, then he must begin the prayer anew."