Why does one perform two prostrations for forgetfulness (sujood as-sahw)? Is there a reason for that, or is it a Sunnah from the Messenger of Allah, may Allah's peace and blessings be upon him?

Prostration of forgetfulness (sujood as-sahw) is a Sunnah (definition) firmly established from the Prophet (peace be upon him), and its wisdom lies in humiliating Satan and compensating for deficiencies in prayer. Al-Nawawi said: "They are a source of anger and humiliation for Satan, who confused the worshiper, so Allah made them a compensation for the prayer and a humiliation for Satan." Ibn al-Qayyim said: "They are a humiliation for Satan in his whispers, and for this reason, the Prophet called them 'the two humiliators'." Al-Shawkani said: "If he prays five [rak'ahs], his prayer will be interceded for him, and even if they are a humiliation for Satan, there is in them a reward that humiliates Satan." Ibn Taymiyyah said: "They take the place of one rak'ah of the prayer, so they intercede for the fifth that was added out of forgetfulness, or they humiliate Satan if the prayer was complete."