^^ Its not really a tajweed rule, actually it is a tajweed exception, where a single length dhamma on the end of the word at'amah*u* followed by inantum illa fi is pronounced as a triple length dhamma. There should be a waw madda in this area as a hint for this dhamma on the ha to be stretched.
I am not however, clear on the reason why this is the case, it could be because simply it is just the rule exception for that particular recitation or it could have a meaning linked with the meaning of the verse. I was hoping you could tell me why.
I knw few Tajweed rules which i hav learnd myself.
1=Ikhfa
it will be pronounced with a light nasal sound.
2=Gunnah
The sound emanantes from the nose and is observed on the NooN & MeeM
3=Ikhfa Meem Sakin
when the lettr Ba appears aftr a Meem it will be Pronunced with a light sound in the nose.
4=Idgham
If aftr a NooN //(Fatah)-----------------------"(Dama) there appear anyof
-----------------------------//(Kasra)
these letters (Wao,Meem,Noon,Ya) it will become assimilated into the lettr and will be read with Ghunna
5=Iqlab
If after a Noon or //(Fatah)-----------------------"(Dama) the lettr Ba
----------------------------------//(kasra)
appears then the Noon saakin or Tanween will be incorporated into the letter Meem and will be recited with Ghunna.
6=Qalqala
the five letters of Qalqala are (daal,Jeem,Ba,Tua,Qaaf) when any of these letters in a word has a sukoon on it or if deciding on pausing on any of these letters which appear at the end of a sentence it will appear to have an echoing or jerking sound.
7=Idghaam Meem saakin
if aftr a Meem there appear another Meem the two meems will become incorporated and will be read with Ghunna.