Sahih Bukhari Volume 001, Book 011, Hadith Number 620.
Narated By Abu Huraira : Allah's Apostle said, "The reward of the prayer offered by a person in congregation is twenty five times greater than that of the prayer offered in one's house or in the market (alone). And this is because if he performs ablution and does it perfectly and then proceeds to the mosque with the sole intention of praying, then for every step he takes towards the mosque, he is upgraded one degree in reward and his one sin is taken off (crossed out) from his accounts (of deeds). When he offers his prayer, the angels keep on asking Allah's Blessings and Allah's forgiveness for him as long as he is (staying) at his Musalla. They say, 'O Allah! Bestow Your blessings upon him, be Merciful and kind to him.' And one is regarded in prayer as long as one is waiting for the prayer."