What is the ruling on the prayer of someone who faces difficulty in pronouncing some letters or words, such as "al-ḍāllīn" (those who have gone astray), during prayer, despite being able to speak normally outside of prayer, and what should he do?

The person mentioned is suffering from waswasah (whispers/obsessive thoughts), and he must ignore them and proceed with his prayer normally without being concerned about the manner of pronunciation. Scholars have warned against waswasah regarding the articulation points of letters, considering it to be from the deception of Iblis, which takes the worshiper away from the proper etiquette of prayer and distracts him with exaggeration from understanding the recitation. The person afflicted with waswasah should know that khushu' (devotion/humility) is the greatest objective of prayer, and that Allah is merciful to His servants and does not burden a soul beyond its capacity. His prayer is valid as long as he strives to abandon this waswasah.