Does the prayer become invalid if the letters are not pronounced correctly during prayer?

It is obligatory to recite a portion from the Qur'an during prayer. For this obligation to be fulfilled, the Qur'an must be recited correctly and in accordance with its proper rules. A slip of the tongue or mistake by the reciter is called zelletü’l-kâri or lahn.

If, during the recitation in prayer, letters with similar articulation points such as "sin" and "sad" are not pronounced exactly from their proper points, the prayer does not become invalid. Even if there is no similarity in articulation points, but some letters are commonly confused and thus difficult to distinguish, many jurists state that the prayer is not invalidated if such letters are substituted for one another. For example, pronouncing (ض) as (ذ), (د), or (ظ) falls into this category. This is because such cases are considered unavoidable and widespread difficulties (umûm-ı belvâ). However, if letters with different articulation points are substituted for each other and the meaning changes, the prayer becomes invalid. If the correct pronunciation is repeated, the prayer becomes valid. (el-Fetâva’l-Hindiyye, 1/78; Ibn Abidin, Reddü’l-muhtâr, 1/630)

According to the Shafi'i school, if a person who has the ability and means to learn substitutes a letter in Surah al-Fatiha with another letter during prayer, the recitation is not valid. However, if the correct pronunciation is immediately recited afterwards, the prayer becomes valid. In this context, for example, if someone who has the ability and means to learn the articulation of the letter (ض) in the phrase “وَلَا الضَّالِّينَ” in Surah al-Fatiha pronounces it as (ظ), then according to the preferred opinion, the prayer becomes invalid. However, if the person does not have the ability or means to learn the correct pronunciation, the prayer is not invalidated. (Ramli, Nihâyetü’l-muhtâc, 1/481) Mistakes in recitation or articulation in other surahs or verses outside of Surah al-Fatiha do not affect the validity of the prayer.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu