ثُمَّ contains a mushaddad mīm, so ghunnah is necessary whether the following word is «كَلَّا» or «لَتَرَوُنَّهَا».
Do not mix up the inherent ghunnah of a doubled mīm with the three contextual rules for mīm sākinah.
Recite «ثُمَّ كَلَّا» and «ثُمَّ لَتَرَوُنَّهَا», keeping the underlying mīm-ghunnah rule consistent in both cases.
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The mushaddad mīm of thumma
Al-ʿAṣr and At-Takāthur: Applied Ghunnah, Madd, Lām and Waqf
ثُمَّ contains a mushaddad mīm, so ghunnah is necessary whether the following word is «كَلَّا» or «لَتَرَوُنَّهَا».
Do not mix up the inherent ghunnah of a doubled mīm with the three contextual rules for mīm sākinah.
Recite «ثُمَّ كَلَّا» and «ثُمَّ لَتَرَوُنَّهَا», keeping the underlying mīm-ghunnah rule consistent in both cases.