Musnad Ahmad 3:2

Yahya ibn Sa'id narrated to us, from Hisham ibn 'Urwah, my father informed me, that Humran informed him, he said: 'Uthman performed ablution (wudu) upon the stone platform, then said: "I shall certainly narrate to you a hadith I heard from the Messenger of Allah, peace and blessings of Allah be upon him. Were it not for a verse in the Book of Allah, I would not have narrated it to you. I heard the Prophet, peace and blessings of Allah be upon him, saying: 'Whoever performs ablution and perfects the ablution, then enters and prays, he is forgiven for what is between it and the other prayer, until he prays it.'"