Is it obligatory to pronounce the last letter of a word with its correct grammatical vowel ending (i'rab) during recitation in prayer, and what is the ruling on making a mistake in that?

It is permissible to pronounce the letter 'ya' in the word 'Rabbi' (My Lord) with a sukoon (quiescent) or a fatḥa (vowel sound). The word 'al-Aẓīm' (the Great) is an adjective for 'Rabbi' and should grammatically be in the genitive case (khafḍ). However, it is also linguistically permissible to pronounce it in the accusative (naṣb) or nominative (rafʿ) cases, as these variations have been transmitted from the Arabs.

Deliberately mispronouncing the dhikr (remembrance of God) is not appropriate, but it does not invalidate the prayer. Shafi'i jurists have differed regarding mispronunciation in the 'ameen' (amen) by geminating the 'meem', but Imam Al-Nawawi confirmed that it does not invalidate the prayer.

Quiescencing the word 'al-Aẓīm' when connecting it to the next word is not a mispronunciation. Rather, it has a permissible linguistic basis, which is connecting with the intention of pausing, and there are many examples of this in eloquent speech, such as Qunbul's recitation from Ibn Kathir of "And I have come to you from Saba' with certain news," where the hamza in 'Saba'' is quiescent.