It was narrated from Shu'bah, the freed slave of Ibn 'Abbas:

When Ibn 'Abbas (رضي الله عنه) did ghusl in the case of janabah, he would scoop water with his right hand onto his left and wash it seven times before putting it into the vessel. On one occasion he forgot how many times he had scooped water unto his hand, so he asked me (Shu'bah). How many times did I scoop water? I said: I do not know. He said: May you be bereft of your mother! Why don't you know? Then he did wudoo' as for prayer, then he poured water over his head and body. He said: This is how the Messenger of Allah (ﷺ) used to purify himself, i.e., do ghusl.