Sayyiduna Suhayb bin al-Nu'man (may Allah be pleased with him) narrates that the Prophet (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."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Muhammad bin Mus'ab al-Qurqusani, whom Yahya bin Ma'in and others declared da'if (weak), while Ahmad declared him thiqah (trustworthy).