Zayd ibn Thabit reports: ''The Prophet peace be upon him chose a place where he went out at night to pray. Some men saw him doing that and they prayed with him. They came every night to do that. One night, the Prophet peace be upon him did not come out to join them. They started to make some noises, like little coughs, and then they raised their voices, and even threw pebbles at his door. He came out to them angry and said: 'Look you people! You continued doing what you did until I thought it might be made obligatory for you. Pray in your own home, because the best prayer a person can offer is that he offers at home, except for obligatory prayers.'''

What angered the Prophet peace be upon him in this incident is that his companions tried every means to get him to come out to lead them in night prayer, which is not an obligatory one. Some of them resorted to impolite means, first raising their voices and then throwing some pebbles at his door. We need to remember that some of the Prophet’s companions were uncultivated Bedouins who lacked polite manners, until Islam gave them the best manners people could have. They were insisting on doing a voluntary worship in a particular way. This is wrong because it imposes a duty where there is none. Hence the Prophet peace be upon him told them that he feared that this night prayer would become obligatory to them. And if it were made obligatory, it would have applied to all generations of Muslims. Thus, their action would have committed them and others to more than what God wished to assign to them as worship duty.

Source: Internet