Is it recommended for the follower (Ma'mum) to say the supplemental phrase 'mil'a as-sama'...' (filling the heavens, etc.)?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
It is not recommended (not Sunnah) for the follower to say the phrase 'mil'a as-sama' wa mil'a al-ard wa mil'a ma shi'ta min shay'in ba'd' (filling the heavens, the earth, and whatever else You will). This is the apparent position in the Madhhab, stated by Ahmad in narrations by Abu Dawud and others, and is the view of the majority of companions of the school. This is because the Prophet (peace be upon him) restricted his command to the followers to only say, 'Rabbana wa lakal hamd,' indicating nothing else is legislated for them.
Supporting text
A narration from Al-Athram transmitted from Ahmad suggests that what is excluded from the follower behind the Imam is only 'Sami' Allahu liman hamidah,' implying that 'Rabbana wa lakal hamd' and its supplements are affirmed for the follower. This is also the choice of Abu Al-Khattab and the position of Ash-Shafi'i, viewing it as a prescribed remembrance in prayer, akin to other invocations.