Musnad Ahmad 482:102

'Abd al-Rahman narrated to us, Mu'awiyah—meaning Ibn Salih—narrated to us, from Rabi'ah, from Abu Idris al-Khawlani, from 'Uqbah ibn 'Amir, who said: And Abu 'Uthman narrated it to him, from Jubayr ibn Nufayr, from 'Uqbah ibn 'Amir, who said: We were assigned the herding of camels, and when my turn came, I brought them back in the evening. I caught up with the Messenger of Allah (ﷺ) standing and speaking to the people. I caught from his words: "There is no Muslim who performs ablution and performs it well, then stands and prays two rak'ahs, focusing upon them with his heart and his face, except that Paradise becomes mandatory for him." I said: "How excellent this is!" Then a speaker in front of me said: "The one before it was more excellent." I looked, and it was 'Umar ibn al-Khattab. He said: "I saw that you just arrived. He [the Prophet] said: 'There is not one of you who performs ablution and perfects the ablution, then says: I bear witness that there is no god but Allah, and I bear witness 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.'"