What is the ruling on pronouncing the letter "ra" without trilling the tongue, and does the prayer become invalid if one of the words of Al-Fatiha that contains a "ra" is recited without trilling?

The scholars of Tajwid have differed on the characteristic of takrir (repetition/trilling) in the letter ra (ر) into three opinions:

1. Takrir is known in order to be avoided. The meaning of describing the ra with it is that it is capable of it, not that it should be produced. 2. Takrir is an inherent characteristic of the ra, because the tip of the tongue stumbles when pronouncing it, which suggests repetition. 3. Takrir is an inherent characteristic of the letter ra that cannot be completely eliminated, but one must guard against increasing it without completely eradicating it.

The last opinion is the most preferred, and accordingly, one must guard against increasing takrir without completely eradicating it.