Sayyiduna Mahmood bin Labeed (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings of Allah be upon him) came out (from his house) and said: "O people, beware of hidden and concealed shirk (polytheism)." The companions asked, "O Messenger of Allah, what is hidden shirk?" He (peace and blessings of Allah be upon him) said: "When a person stands to pray and beautifies his prayer, and when he sees people looking at him, he puts extra effort and strives (in his prayer), then this is hidden and concealed shirk."