It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) inadvertently performed five rak'ahs instead of four in the Zuhr prayer. Someone asked: Has there been an addition in the prayer that you performed five rak'ahs? Upon this, the Prophet (peace and blessings be upon him) performed two prostrations of forgetfulness (sahw).