Sahih Bukhari Volume 1, Book 4, Hadith 136

حَدَّثَنَا يَحْيَى بْنُ بُكَيْرٍ، قَالَ حَدَّثَنَا اللَّيْثُ، عَنْ خَالِدٍ، عَنْ سَعِيدِ بْنِ أَبِي هِلاَلٍ، عَنْ نُعَيْمٍ الْمُجْمِرِ، قَالَ رَقِيتُ مَعَ أَبِي هُرَيْرَةَ عَلَى ظَهْرِ الْمَسْجِدِ، فَتَوَضَّأَ فَقَالَ إِنِّي سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ ‏"‏ إِنَّ أُمَّتِي يُدْعَوْنَ يَوْمَ الْقِيَامَةِ غُرًّا مُحَجَّلِينَ مِنْ آثَارِ الْوُضُوءِ، فَمَنِ اسْتَطَاعَ مِنْكُمْ أَنْ يُطِيلَ غُرَّتَهُ فَلْيَفْعَلْ ‏"

Narrated By 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).'"