Musnad Ahmad 25:941

Yazid ibn Harun narrated to us, Ibn Abi Dhi’b informed us, from Shu’bah, the freed slave of Ibn Abbas, that when Ibn Abbas would perform ghusl (ritual bath) for janabah (major ritual impurity), he would pour with his right hand onto his left, and wash it seven times before inserting it into the vessel. Once, he forgot how many times he had poured onto his hand, so he asked me, "How many did I pour?" I said, "I do not know." He said, "May you have no mother! Why do you not know?" Then he performed his wudu (ablution) for prayer, and then poured water over his head and his body. He said, "This is how the Messenger of Allah, peace and blessings of Allah be upon him, used to purify himself," meaning he used to perform ghusl.