In Sunnah prayers, is it obligatory to recite only Al-Fatihah, or should one recite whatever else is easy from the Qur'an? And does the ruling differ based on the type of Sunnah prayer (such as the regular Sunnah prayers associated with the obligatory prayers, Witr, or Shaf‘)?

"The jurists are of the view that supererogatory prayers (Nawafil) share the rulings of obligatory prayers (Fara'id) in some matters, including the recitation of Al-Fatiha and additional surahs. This is because the Prophet, peace and blessings be upon him, used to recite Al-Fatiha in every rak'ah of both obligatory and supererogatory prayers. He would add more to Al-Fatiha in supererogatory prayers. For instance, in the two rak'ahs of Fajr and the regular Sunnah of Maghrib, he would recite Al-Kafirun and Al-Ikhlas. In Witr prayer, he would recite 'Glorify the name of your Lord, the Most High,' 'Say, "O disbelievers,"' 'Say, "He is Allah, the One,"' and the two Mu'awwidhatayn (Al-Falaq and An-Nas). In the Eid prayers and Jumu'ah prayer, he would recite 'Glorify the name of your Lord, the Most High' and 'Has there come to you the narration of the overwhelming event?' This indicates the congruence between obligatory and supererogatory prayers in reciting Al-Fatiha and something after it in all their rak'ahs, except that the addition after Al-Fatiha in obligatory prayers is limited to the first two rak'ahs according to many scholars."