Hadith a day – no. 44

Narrated By ‘Abdullah: I Used to greet the Prophet while he was in prayer
and he would return my greeting, but when we returned (from Ethiopia) I
greeted the Prophet (while he was praying) but he did not return the
greeting, and (after finishing the prayer) he said, “In the prayer one is
occupied (with a more serious matter).”

Bukhari – Volume 2 – Book 22 – Hadith 307