It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) led five rak'ahs in Zuhr or Asr prayer. When it was realized, he performed two prostrations of forgetfulness (sujood as-sahw) and said, "These two prostrations are for the one who fears any deficiency or excess in his prayer."