Silsilat al Huda wan Noor Tape 9, Question 9
The voluntary prayer of a man (in a place) where no person can see him equal to twenty-five times his prayer where people can see him. 
The superiority of the prayer of a man in his house over his prayer (in a place) so that people can see him, is like the superiority of the obligatory prayer over the voluntary 
 Musnad Aboo Ya’ala, Hadeeth 3821. Shaykh Al-Albanee graded this narration as authentic.
 Saheeh At-Targeeb Wa At-Tarheeb, The Book Of Desiring to Pray The Voluntary Prayers in the House, Hadeeth 438. Shaykh Al-Albanee graded this narration as Hasan.