Abu Sa'id Khudri (may Allah be pleased with him) narrates: The Messenger of Allah (peace and blessings be upon him) came to us while we were discussing the Messiah Dajjal. He (peace and blessings be upon him) said: "Shall I not tell you about something concerning which I fear for you even more than the Messiah Dajjal?" We said: O Messenger of Allah! Please do tell us. He (peace and blessings be upon him) said: "Hidden shirk: it is when a man is performing prayer, and when he sees that someone is watching him, he lengthens his prayer (making it longer with slow recitation to show off)." Hasan, narrated by Ibn Majah.