Does thinking during prayer invalidate it?

Question:
If I am in prayer and it occurs to me after I have completed the prayer that I should do a certain task, but I did not change anything in the course of my prayer, is my prayer valid? And is there anything required of me?

Answer:
The prayer is valid, but a person should drive away whispers and distractions from his heart and return to his prayer. If he does not drive them away and continues with the distractions and the whispers, then this diminishes his reward; it diminishes the reward. A person gets from his prayer only what he understood of it and what his heart was present for. This is the humility (khushu') of which Allah, Glorified and Exalted is He, said: So the Muslim should try to be humble before his Lord, to keep his heart present, to cut off the whispers, and to cut off the distractions from his mind and his heart, and to devote himself to his prayer so that its reward is recorded for him. Yes.