Question:

Is it from the sunnah for the one who enters the masjid to give the salaams whether there is anyone in it or not, because we heard that whoever enters the masjid it is a must that he gives salaams upon the angels?

Answer:

Give the salaams if someone is in it, (if) he sees someone in it then give the salaams. As for if the masjid is empty then there is no reason to give the salaams.

Source: http://www.alfawzan.af.org.sa/node/14495

[youtube http://www.youtube.com/watch?v=y7skW6NOAh0?rel=0&w=560&h=315%5D