Musnad Ahmad 3:32

Ishaq ibn Yusuf narrated to us, 'Awf al-A'rabi narrated to us, from Ma'bad al-Juhani, from Humran ibn Aban, who said: We were with Uthman ibn Affan, and he called for water and performed ablution. When he finished his ablution, he smiled and said: "Do you know why I smiled?" He said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed ablution just as I have performed ablution, then he smiled and said: 'Do you know why I smiled?'" We said: "Allah and His Messenger know best." He said: "When the servant performs ablution and perfects his ablution, then enters into his prayer and perfects his prayer, he emerges from his prayer free of sins, just as he emerged from his mother's womb."