Musnad Ahmad 482:72

Abdullah ibn Yazid narrated to us, from Sa'id ibn Abi Ayyub, Zuhrah ibn Ma'bad narrated to me, from a cousin of his, the brother of his father, that he heard Uqbah ibn Amir saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs wudu and perfects his wudu, then raises his gaze to 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."