Musannaf Ibn Abi Shayba 21:775

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