Musnad Abu Ya'la al-Mawsili 152:49

Muhammad ibn Abi Bakr narrated to us, Muhammad ibn Dinar narrated to us, from Ibrahim al-Hajari, from Abu al-Ahwas, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever beautifies the prayer where people see him, and makes it ugly where he is alone, then that is a belittlement with which he has belittled his Lord, Blessed and Exalted is He."