Answered by Shaykh Faraz Rabbani
if you are praying behind imam and mistakenly recite a few letters of say fatiha with you tonguesuch as ‘leen’ or even just the ‘l’, is the prayer invalidated and requiring make up?
Answer:
Assalamu alaikum wa rahmatullah
I pray this finds you in the best of health and spirits.
Forgetfully doing something disliked behind the imam (such as reciting) is excused, and no redress or correction is required for it. [ Ibn Abidin, Radd al-Muhtar ]
Jazakum Allahu Khayr
Wassalam