Musannaf Ibn Abi Shayba 1:24

Al-Muqri narrated to us, from Sa’id ibn Abi Ayyub, who said: Zahrah ibn Ma’bad Abu ‘Aqil narrated to me, that a cousin of his informed him that he heard ‘Uqbah ibn ‘Amir saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs ablution and perfects his ablution, then raises his head toward the sky and says: 'I bear witness that there is no god but Allah alone, with no partner, and that Muhammad is His servant and His Messenger,' the eight gates of Paradise will be opened for him; he may enter through whichever of them he wishes."