Musnad Abu Ya'la al-Mawsili 2:41

Al-Qawariri narrated to us, Abdullah bin Yazid narrated to us, Haywah bin Shurayh narrated to us, Abu Aqil narrated to us, from a cousin of his, from Uqbah bin Amir, that he was with the Messenger of Allah (may Allah bless him and grant him peace). Umar said: The Messenger of Allah (may Allah bless him and grant him peace) said before he arrived: "Whoever performs ablution and performs the ablution well, then raises his sight to the sky and says: 'I bear witness that there is no god but Allah alone, with no partner, and I bear witness 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."