Sahih Bukhari Volume 2, Book 22, Hadith Number 290.

Narated By ‘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).”