Shuba reported,

Whenever Ibn Abbas performed the ritual bath after major impurity, he would pour water with his right hand onto his left and wash it seven times before dipping it into the vessel.One time he forgot how many times he had poured water over his hand, so he asked me, "How many times did I pour?"I replied, "I do not know."He said, "May you be motherless, why do you not know?"Then he performed wudu as he would for prayer, after which he poured water over his head and body. He said, "This is how the Messenger of Allah ﷺ used to purify himself," meaning, perform the ritual bath.

حدثنا يزيد بن هارون أخبرنا ابن أبي ذئب

عن شعبة مولى ابن عباس

أن ابن عباس كان إذا اغتسل من الجنابة أفرغ بيده اليمنى على اليسرى فغسلها سبعا قبل أن يدخلها في الإناء فنسي مرة كم أفرغ على يده فسألني كم أفرغت فقلت لا أدري فقال لا أم لك ولم لا تدري ثم توضأ وضوءه للصلاة ثم يفيض الماء على رأسه وجسده قال هكذا كان رسول الله ﷺ يتطهر يعني يغتسل