Volume 1, Book 4, Number 138:

Narrated Nu'am Al-Mujmir: Once I went up the roof of the mosque, along with Abu Huraira. He perform ablution and said, "I heard the Prophet saying, "On the Day of Resurrection, my followers will be called "Al-Ghurr-ul-Muhajjalun" from the trace of ablution and whoever can increase the area of his radiance should do so (i.e. by performing ablution regularly).' "