Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) came out and said: "O people! Beware of hidden shirk." The Companions (may Allah be pleased with them) said: O Messenger of Allah! What is hidden shirk? He (peace and blessings be upon him) said: "That a man stands for prayer and strives to beautify his prayer so that people may see him; that is hidden shirk."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3585