Answer
by Sheikh Saalih al-Uthaimeen رحمه الله:
There is no doubt that supplication has an
effect on changing what is written, but
this change was also written because of the supplication. So, do not think that if
you supplicate to Allaah سُبحانه
وتعالى that you are supplicating
with something which was not written; rather the supplication is written,
and what results from it is written. This is why we find that the reciter
recites over the sick person and he is cured, and in the story of the military
expedition which the Prophet صلّى اللَّهُ عليه وسلّم sent,
they stayed as guests with a people, but they did not treat them hospitably,
and it was decreed that a snake bit their leader, so they requested someone to
recite something over him. The Companions رضي الله عنهم
imposed
the condition that they should be paid for that, and they gave them a flock of
sheep, so one of them went and recited Surat al-Faatihah over him, and the man
who was bitten arose as if he had been freed from shackles, that is, as if he
was a camel which had broken free from its tethering rope. Therefore the
recitation had an effect in curing the sick person.