It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever performs the prayer well in order to show people, and does not perform it well when alone, that is an act of contempt and mockery towards his Lord."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3584