"In the prayer one is occupied (with a more serious matter)."

SAHIH BUKHARI - Volume 2, Book 22, Number 290:
Narrated 'Abdullah:

We used to greet the Prophet while he was praying and he used to answer
our greetings. When we returned from AnNajashi (the ruler of Ethiopia),
we greeted him, but he did not answer us (during the prayer) and (after
finishing the prayer) he said, "In the prayer one is occupied (with a
more serious matter)."