Then, Amir al-Mu'minin continues: "When you say the opening Takbir, recite Surah al-Hamd (the Opening Chapter) and any other Surah that you wish, except for four specific combinations: 'Surah al-Duha' and 'Surah Alam Nashrah' as they are one Surah together, and 'Surah al-Fil' and 'Surah Quraysh' as they are also one Surah together. If you choose to read them, both pairs must be read in the same unit (rak‘ah). Do not recite just one of these in a single rak‘ah of a mandatory prayer." Further, he added: "Do not combine two Surahs in one unit of a mandatory prayer. In optional (nafl) prayers, you may combine as you wish. Also, do not recite any of the four 'prostration' (al-'Azā'im) chapters in the mandatory prayers, namely Surah Sajda (31), Surah Ha Mim Sajda (41), Surah al-Najm (53), and Surah al-‘Alaq (96). If you recite any of these, you must prostrate, saying: 'O’ my God, we have believed in what they disbelieved in, and we have recognized from You what they denied, and we have answered Your call, O’ my God, so forgive, forgive.' Then you may rise and continue with your prayer."
Commentary #4