Sunan Ibn Majah — The Book of Purification and its Sunnah — Hadith No: 419
Translation
It was narrated that Ibn 'Umar said:"The Messenger of Allah performed ablution washing each part once. He said: 'This is the ablution of the person from whom Allah will not accept his prayer without it.' Then he performed ablution washing each part twice, and he said: 'This is the ablution that Allah appreciates.' Then he performed ablution washing each part three times, and said: 'This is how ablution is performed properly, and this is my ablution and the ablution of the Close Friend of Allah, Ibrahim. Whoever performs ablution like this, then on completing it says: 'Ashhadu an la ilaha illallah, wa ashhadu anna Muhammadan 'abduhu wa rasuluhu' (I bear witness that none has the right to be worshipped but Allah, and I bear witness that Muhammed is His servant and His Messenger), eight gates of Paradise will be opened to him and he may enter through whichever one he wants