Musnad Ahmad 482:24

Abu al-Ala al-Hasan ibn Siwar narrated to us, he said: Layth narrated to us, from Mu’awiyah, from Abu Uthman, from Jubayr ibn Nufayr and Rabi’ah ibn Yazid, from Abu Idris al-Khawlani and Abd al-Wahhab ibn Bakht, from al-Layth ibn Sulaym al-Juhani, all of them narrating from Uqbah ibn Amir. Uqbah said: We used to serve ourselves, and we used to take turns tending the camels among us. My turn to tend the camels came, so I brought them back in the evening. I caught up with the Messenger of Allah (peace and blessings of Allah be upon him) while he was standing and speaking to the people. I caught part of his speech, and he was saying: "There is no one among you who performs ablution and perfects the ablution, then stands and prays two rak'ahs, focusing upon them with his heart and his face, except that Paradise becomes mandatory for him and he is forgiven." He said: I said, "How excellent this is!" He said: A speaker in front of me said, "The one that was before it, O Uqbah, is more excellent than it." I looked, and it was Umar ibn al-Khattab. He said: I said, "And what is it, O Abu Hafs?" He said: "He said, before you arrived: 'There is no one among you who performs ablution and perfects the ablution, then says: I bear witness that there is no god but Allah alone, with no partner, and that Muhammad is His servant and His Messenger, except that the eight gates of Paradise are opened for him; he may enter through whichever of them he wishes.'"