From Sahih Bukhari (Hadith 882):
Narrated Abu Huraira:While `Umar (bin Al-Khattab) was delivering the Khutba on a Friday, a man entered (the mosque). `Umar asked him, "What has detained you from the prayer?" The man said, "It was only that when I heard the Adhan I performed ablution (for the prayer)." On that `Umar said, "Did you not hear the Prophet saying: 'Anyone of you going out for the Jumua prayer should take a bath'?
Reference: Bukhari 882