Tajweed RulesOne of the rulings of the sakinah mim: if the letter ba follows it, it must be concealed with a labial concealment (ikhfa shafawi), with ghunnah for the duration of two harakahs. It is called 'labial' because both mim and ba are articulated from the two lips.
تَرْمِيهِم بِحِجَارَةٍ — وَهُم بِالْآخِرَةِSource:تحفة الأطفال
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If a moving (voweled) mim follows the sakinah mim, the first is merged into the second with ghunnah for the duration of two harakahs, becoming a single doubled (shaddah)…
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If any letter other than ba and mim follows the sakinah mim, the mim must be pronounced with a clear labial idhhar, without ghunnah.
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The madd (prolongation) without which the letter itself cannot stand, and which does not depend on a hamzah or a sukun; it is prolonged for the duration of two harakahs.