Musnad Ahmad 27:1285

Aswad ibn Amir narrated to us, Israel informed us, from Zayd al-Ammi, from Nafi’, from Ibn Umar, from the Prophet (peace be upon him), who said: "Whoever performs wudu once, that is the duty of wudu which is indispensable. Whoever performs wudu twice, he has two portions. And whoever performs wudu three times, that is my wudu and the wudu of the prophets before me."