Sunan Ibn Majah, Chapter: The Chapters on the Mosques and the Congregations

"The Messenger of Allah (saw) said: 'When one of you performs ablution and does it well, then he comes to the masjid (mosque) with no other motive but prayer and not seeking anything other than prayer, he does not take one step but Allah raises him in status one degree thereby, and takes away one of his sins, until he enters the masjid (mosque). When he enters the masjid (mosque) he is in a state of prayer so long as he is waiting for the prayer.'" (Sahih)