Tafkhīm is the prescribed full resonance and tarqīq the lighter sound. A madd alif generally follows the sound quality of the preceding letter; do not give all alifs the same heaviness.
For the lām in the divine name:
وَاللّٰهِ — the preceding fatḥah calls for a heavy lām.
بِاللّٰهِ — the preceding kasrah calls for a light lām.
The pronunciation rule is phonetic, not a difference in respect. Both expressions contain Allah's name, while the preceding vowel changes the lām's required quality.
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Tafkhīm and tarqīq: alif and divine-name lām
Comprehensive Revision — Applying Tajwīd Rules
Tafkhīm is the prescribed full resonance and tarqīq the lighter sound. A madd alif generally follows the sound quality of the preceding letter; do not give all alifs the same heaviness.
For the lām in the divine name:
وَاللّٰهِ — the preceding fatḥah calls for a heavy lām.
بِاللّٰهِ — the preceding kasrah calls for a light lām.
The pronunciation rule is phonetic, not a difference in respect. Both expressions contain Allah's name, while the preceding vowel changes the lām's required quality.