Musnad Ahmad 962:26

Muhammad ibn Bishr narrated to us, Sa'id ibn Abi 'Aruba narrated to us, from Qatadah, from Shahr ibn Hawshab; and 'Abd al-Wahhab narrated to us, from Hisham and Azhar ibn al-Qasim, Hisham narrated to us, from Qatadah, from Shahr ibn Hawshab, from Abu Umamah, the companion of the Messenger of Allah, may Allah bless him and grant him peace. And 'Abd al-Wahhab said: Abu Umamah al-Humsi, the companion of the Messenger of Allah, may Allah bless him and grant him peace, that the Messenger of Allah, may Allah bless him and grant him peace, said: "Wudu (ablution) expiates what came before it, then the prayer becomes a supererogatory act." It was said to him: "Did you hear it from the Messenger of Allah, may Allah bless him and grant him peace?" He said: "Yes, not once, nor twice, nor three times, nor four times, nor five times."