When the Messenger of Allah (may Allah's peace and blessings be upon him) took a ritual bath to remove Janābah, he would start by washing his hands. Then he would pour water with his right hand over his left hand to wash his private parts. Then he would make ablution like that he would make for his prayer
Authentic hadithعن عائشة، قالت: «كان رسول الله صلى الله عليه وسلم إذا اغتسل من الجَنَابَة يبدأ فيغسل يديه، ثم يُفرغ بيمينه على شماله فيغسل فَرْجَه، ثم يتوضأ وضوءه للصلاة، ثم يأخذ الماء فيُدخل أصابعه في أصول الشَّعَرِ، حتى إذا رأى أن قد اسْتَبْرَأَ حَفَنَ على رأسه ثلاث حَفَنَات، ثم أفاض على سائر جسده. ثم غسل رجليه».
‘Ā'ishah (may Allah be pleased with her) reported: “When the Messenger of Allah (may Allah's peace and blessings be upon him) took a ritual bath to remove Janābah, he would start by washing his hands. Then he would pour water with his right hand over his left hand to wash his private parts. Then he would make ablution like that he would make for his prayer. Then he would take water with his fingers and run them through his hair. When making sure that his hair was thoroughly wet, he would pour three handfuls of water over his head. Then he would pour water over the rest of his body. Then he would wash his feet.
Explanation
‘Ā'ishah (may Allah be pleased with her) describes how the Prophet (may Allah's peace and blessings be upon him) took a ritual bath. He would start with his hands, when he wanted to wash from Janābah. He would clean his hands first because he would scoop water with them. Then he would make an ablution typical of his regular ablution for prayer. Since the Prophet (may Allah's peace and blessings be upon him) had thick hair, he would run his fingers through it with water. When water reached the roots of the hair, he would pour water over his head three times. Then he would wash the rest of his body. He would finally wash his feet.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).