Making up missed sunnahs

The evidence is his saying (peace and blessings of Allah be upon him):

When one of you stands to pray, Satan comes to him and confuses him until he no longer knows how much he has prayed; when one of you finds that, let him prostrate twice while sitting.

Narrated by al-Bukhari (1175) and Muslim (389)

His saying "stands to pray" encompasses all prayers, obligatory and voluntary: the text is general for everything called prayer. Ibn Abi Shaybah related through his chain from Abu Aqil that he heard Sa'id ibn al-Musayyab say: "The 2 prostrations of forgetfulness in the voluntary prayers are like the 2 prostrations of forgetfulness in the obligatory prayer" (al-Musannaf of Ibn Abi Shaybah, 4433; al-Awsat, 3/336; al-Mudawwanah, 1/137; al-Hidayah, 1/52; al-Zarqani, 1/105; al-Majmu', 4/161; al-Mughni, 1/698; Umdat al-Qari, 7/303; Sharh Ibn Battal, 3/230; Sunan al-Bayhaqi al-Kubra, 1/137).

The ruling on making up sunnahs: the scholars differed over the legitimacy of making up the rawatib sunnahs in 4 views:

• The first view: making them up is recommended absolutely, whether the omission was with an excuse or without; to this went al-Shafi'i in the newer position, Ahmad and Muhammad ibn al-Hasan.
• The second view: they are not made up; this is the view of Abu Hanifah, Malik and Abu Yusuf according to the more famous of the 2 narrations from him, the view of al-Shafi'i in the older position, and a narration related from Ahmad; the famous position from Malik is making up the 2 rak'ahs of Fajr after sunrise.
• The third view: distinguishing between what is independent in itself, such as the Eid and Duha prayers, which are made up, and what follows something else, such as the rawatib of the obligatory prayers, which are not made up; this is one of the views related from al-Shafi'i.
• The fourth view: that he makes them up if he wishes and omits them if he wishes, by way of free choice; it is related from the rationalists (ashab al-ra'y) and Malik (Nayl al-Awtar, 3/31; al-Hidayah wa-l-'Inayah, 1/342; Sharh Fath al-Qadir, 1/479; al-Binayah, 2/613; Sharh Mukhtasar Khalil of al-Kharashi, 2/16; al-Wasit, 2/217; Rawdat al-Talibin, 2/78; al-Majmu', 4/45; al-Munthur, 3/74; al-Fatawa al-Fiqhiyyah al-Kubra, 1/186; Majmu' al-Fatawa, 22/299; al-Insaf, 2/178).