Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1355
Abu Dharr reported: My friend (the Holy Prophet) bade me to hear and obey (the ruler) even if he is a slave having his feet and arms cut off, and observe prayer at its prescribed time. (And further said): It you find people having observed the prayer, you in fact saved your prayer, otherwise (if you join with them) that would be a Nafl prayer for you.