It is narrated from Sayyiduna Suhayb bin al-Nu'man (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The superiority of a man's prayer in his house over his prayer where people can see him is like the superiority of the obligatory prayer over the voluntary prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7291