It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) sent me for a task of his. I went, and when I returned after completing that task, I greeted the Prophet (peace and blessings be upon him) with salam, but he did not reply. Then I saw him bowing and prostrating, so I stepped back. Then the Prophet (peace and blessings be upon him) said: "Did you complete the task?" I replied: "Yes, I did it in such and such a way." The Prophet (peace and blessings be upon him) said: "Nothing prevented me from replying to you except that I was performing prayer."