It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that once the Holy Prophet (peace and blessings be upon him) by mistake led two rak'ahs of Zuhr prayer and ended the prayer with salam. The companions (may Allah be pleased with them) asked, "Has the prayer been shortened?" Upon this, the Holy Prophet (peace and blessings be upon him) stood up, added two more rak'ahs, and after ending the prayer with salam, performed two prostrations of forgetfulness (sujood as-sahw).