Musnad Ahmad 27:1180

Yahya ibn Adam narrated to us, Zuhayr narrated to us, from Musa ibn Uqbah, from Salim ibn Abdullah ibn Umar, from Abdullah ibn Umar, regarding the dream of the Messenger of Allah, peace and blessings of Allah be upon him, concerning Abu Bakr and Umar. He said: "I saw the people gathered, then Abu Bakr stood up and drew a bucket or two, and there was weakness in his drawing, and may Allah forgive him. Then Ibn al-Khattab stood up, and it transformed into a large bucket, and I have never seen a genius among the people performing his feat, until the people watered their camels and rested them."