Musannaf Ibn Abi Shaybah

Hazrat Ibn Buraidah narrates that a man sought permission from one of the companions of the Holy Prophet (peace and blessings be upon him) while he was standing at the door. That person said three times, "May I come in?" He was looking at him but did not grant him permission. Then he asked them like this: "Assalamu Alaikum, may I come in?" He said to him: "Come in." Then he said: "If you had kept standing the whole night saying, 'May I come in?' I would not have granted you permission until you began with Salam."