What is the ruling if one stands up for a third rak'ah during the night prayer (Qiyam al-Layl)?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If one stands up during the night prayer (Qiyam al-Layl) as if intending a third rak'ah, the ruling is the same as standing for a third rak'ah in the Fajr prayer (which requires sitting). Ahmad explicitly stated this ruling. The ruling is based on the Prophet's statement: "The night prayer is in sets of two (Rak'ah)." Since the prayer was legislated as two rak'ahs, its ruling follows the established ruling for the Fajr prayer.
Supporting text
Malik held that the prayer should be completed as four rak'ahs, and prostration for forgetfulness should be performed, whether the mistake occurred at night or during the day. Al-Shafi'i in Iraq held the same view as the main opinion stated above. Al-Awza'i stated that in the daytime prayer, the ruling is the same as the main opinion (sitting), but in the night prayer, if one remembers before bowing in the supposed third rak'ah, one sits and performs prostration for forgetfulness; but if one remembers after bowing, one completes it as four rak'ahs.