Recitation in obligatory and supererogatory Prayers

Q 4: There is an Imam (the one who leads congregational Prayer) who recites the Saying of Allah (Exalted be He): (Part No. 5; Page No. 310) ...and say: "My Lord! Bestow on them Your Mercy as they did bring me up when I was young." in the Jahri Salah (Prayer recited out loud) then says: “My Lord! Bestow on them Your Mercy as they did bring me up when I was young.” And when he recites: Say (O Muhammad صلى الله عليه وسلم): "He is Allâh, (the) One , he says: “He is Allah, the One” then continues reciting, what is the ruling on this deed?

A: If he does so in obligatory Salah (Prayer), then it is Sunnah to abandon this deed; because, as far as we know, it is not authentically reported from the Prophet (peace be upon him) that he did so. But if the reciter does so in a supererogatory Salah such as Tahajjud (optional late night Prayer), it is desirable for him that when he recites an Ayah (Qur’anic Verse) about Mercy to ask Allah for Mercy and when he recites an Ayah about torture, to ask Allah to protect him from torture and when he recites an Ayah about Tasbih (glorification of Allah) to do Tasbih (saying: "Subhan Allah [Glory be to Allah]"). Because it is related by Muslim in (Sahih Muslim) That Hudhayfah (may Allah be pleased with him) offered Salah with the Prophet (peace be upon him) one night then described the recitation of the Prophet (peace be upon him) saying he used to recite leisurely and if he recited an Ayah which mentioned Tasbih (glorification of Allah), he would glorify Allah; if he recited an Ayah which mentioned begging, he would beg from Him; if he recited an Ayah which mentioned seeking refuge with Allah, he would seek refuge with Him. As for the deed of some Imams who, on reciting: Say (O Muhammad صلى الله عليه وسلم): "He is Allâh, (the) One , they say: “He is Allah, the One”, it is not permissible. In fact, it is Tasbih that is permissible in Nafilah (supererogatory) Prayer as we mentioned before.