Answered by Shaykh Faraz Rabbani
Is it permissible to pause in between al-fatiha and the following surah in salaat when thinking of what ayah to recite? How long a pause would invalidate one’s prayer?
Answer:
Walaikum assalam, wa rahmatullahi wa barakatuhu,
Even a long pause would not invalidate the prayer. However, pausing excessively (= the extent of saying SubhanAllah three times) after the inevitable pause expected for thought, catching one’s breath, or some action of the prayer would render the prayer prohibitively disliked (makruh tahriman).
And Allah knows best.
Faraz Rabbani