Silsilat al-Ahadith al-Sahihah

Sayyiduna Anas bin Malik (may Allah be pleased with him) says: I used to serve the Messenger of Allah (peace and blessings of Allah be upon him) and would go to him without permission. One day I came and (as usual) went straight inside. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Son! Go back. A new command has been enforced; (from now on) do not enter upon me without permission."