1. In the recitation, it is obligatory to pronounce the words in such a way that it is truly considered a recitation; therefore, silent recitation in the heart-merely passing the words through the mind without verbal articulation-is not sufficient.
2. A person who is mute and unable to speak may perform the prayer using gestures, and such a prayer is valid.
3. A person must recite the prayer correctly and without mistakes. However, if someone is completely unable to learn the correct recitation, they should pray in whatever manner they can. It is recommended, as a matter of precaution, that such a person perform the prayer in congregation.
4. The criterion for the validity of recitation is the correct articulation of the letters — observing their proper vowels, pauses, and points of pronunciation — in such a way that native Arabic speakers would recognize the sound as that specific letter and not another. Observing the finer rules of tajwīd (such as beautification techniques) is not obligatory.

Ayatollah Makarem Shirazi: Tajwīd details are not obligatory, but one should recite in a way that words are correctly pronounced and recognizable.

5. If a person has been mispronouncing the recitation or the dhikr in prayer – for example, pronouncing “yūladu” with a kasrah instead of a fatḥah on the letter lām – then if they were a negligent ignorant (jāhil muqaṣṣir), their prayer is, as a matter of obligatory precaution, invalid.
However, if they were a blameless ignorant (jāhil qāṣir) and recited it in that manner believing it to be correct, their prayer is valid.
6. Connecting one verse to the next in prayer without pronouncing the final vowel of the first verse does not invalidate it. For example, if a person joins two verses during recitation, it is not necessary to pronounce the final vowel of the first verse. For instance, saying “Māliki yawmid-dīn” with a silent nūn at the end, and immediately continuing with “Iyyāka na‘budu wa iyyāka nasta‘īn”, is perfectly acceptable.

Ayatollah Sistani: it is recommended as a precaution (iḥtiyāṭ mustaḥabb) in prayer neither to pause on a vowel (waqf bi-ḥarakah) nor to connect a word to the next with a silent ending (waṣl bi-sukūn).

Waqf bi-ḥarakah means pronouncing the final vowel (fatḥah, kasrah, or ḍammah) of a word and then making a slight pause before continuing to the next word.

For example: “Ar-Raḥmān ir-Raḥīm” – pronouncing the mīm of ir-Raḥīm with a kasrah, pausing slightly, and then continuing: “Māliki yawmid-dīn.”
Waṣl bi-sukūn means not pronouncing the final vowel of a word and immediately connecting it to the next word. For example: “Ar-Raḥmān ir-Raḥīm” – leaving the mīm of ir-Raḥīm without a kasrah and immediately continuing: “Māliki yawmid-dīn.”

Recommended Acts of Recitation (Mustahabbāt al-Qirā’ah)

1. It is recommended that in the first rak‘ah of prayer, before reciting Sūrat al-Fātiḥah, one says: “A‘ūdhu billāhi min ash-shayṭān al-rajīm.”
   Also, in the first and second rak‘ahs of the Ẓuhr and ‘Aṣr prayers, one should recite “Bismillāh al-Raḥmān al-Raḥīm” aloud.
   It is also recommended to recite Sūrat al-Fātiḥah and the following surah slowly and distinctly, pausing briefly at the end of each verse.
   After finishing Sūrat al-Fātiḥah — whether praying in congregation or individually, and whether as imam or follower — it is recommended to say: “Al-ḥamdu lillāhi rabb al-‘ālamīn.”
   Then, after reciting Sūrat Qul Huwa Allāhu Aḥad, it is recommended to say once or twice: “Kathālika Allāhu rabbī.”
2. It is recommended that in the third and fourth rak‘ahs, after reciting the tasbīḥāt, one seeks forgiveness — for example, by saying: “Astaghfirullāha rabbī wa atūbu ilayh” or “Allāhumma ighfir lī.”
3. It is recommended in all prayers to recite Sūrat al-Qadr in the first rak‘ah and Sūrat al-Ikhlāṣ in the second.
4. It is considered makrūh (disliked) to omit Sūrat al-Ikhlāṣ in any of the daily prayers.
   It is also makrūh to repeat the same surah in both rak‘ahs of one prayer, except for Sūrat al-Ikhlāṣ.