Why does the word for prayer (صلاة) appear in the singular in the second verse of Surat Al-Mu'minoon: "Those who are humble in their prayers," while it appears in the plural in the ninth verse: "And those who guard their prayers"? And is it permissible to read "salawatuhum" (their prayers) in the singular form?

Ibn Ashour indicated that the plural form of "salawat (definition)" (prayers) in the ninth verse signifies the importance of observing all their prescribed numbers, whereas its singular form at the beginning of the surah (definition) denotes the general concept of prayer, as devotion is required in every prayer. It is permissible to read "salatihim" (their prayer) in the singular for those who follow the recitation of Hamzah, Al-Kisa'i, and Khalaf.