What is the proof that we don’t have to smile on light letters of Arabic in Quran and in real life and also pronouncing heavy letters heavily in both scenarios? Please, could you quote a scholar like bin baz.
Wa alikom asslam wa rahmatu Allahi taala wa barakatuh,
Do you mean a proof of a tajweed rule? A proof for tajweed rules in general and how the Quran was recited is part of the Quran since it was learned directly from the Prophet (peace be upon him), that’s why we learn the tajweed and the Quran from sheikhs face to face, and they would hear from us, and correct the mistakes if there are any. And they in turn learned it from their sheikhs, and so on ..
However, if you mean a proof for following the tajweed rules in general, and whether it is mandatory, then you can check out this audio fatwa of shaikh ibn baz: CLICK HERE
Hope this answers the question.
Hope this answers the question in shaa Allah.Allah knows best.