Musnad Ahmad 32:1848

Mu'awiyah ibn 'Amr narrated to us, Za'idah narrated to us, 'Amr ibn 'Abd Allah ibn Wahb narrated to us, Zayd al-'Ammi narrated to us, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, who said: "Whoever performs wudu and perfects the wudu, then says three times: 'I bear witness that there is no god but Allah alone, He has no partner, and that Muhammad is His servant and His messenger,' the eight gates of Paradise are opened for him; he may enter through whichever of them he wishes."