Hadith: It is sufficient for you to pour three handfuls of water on your head then pour water all over yourself, and you will be purified

عن أم سلمة رضي الله عنها قالت: قلت: يا رسول الله، إنِّي امرأة أَشُدُّ ضَفْرَ رأسي فَأَنْقُضُهُ لغُسل الجَنابة [وفي رواية: والحَيْضَة]؟ قال: «لا، إنَّما يَكْفِيك أن تَحْثِي على رأْسِك ثلاث حَثَيَاتٍ ثم تُفِيضِينَ عليك الماء فَتَطْهُرين».
[صحيح] - [رواه مسلم]

Umm Salamah (may Allah be pleased with her) reported: I said: "O Messenger of Allah, I am a woman who braids her hair. Should I undo it when I wash from Janābah – in another narration: and after menstruation?" He said: "No, it is sufficient for you to pour three handfuls of water on your head then pour water all over yourself, and you will be purified."
[Authentic hadith] - [Narrated by Muslim]

Explanation

Umm Salamah (may Allah be pleased with her) reports that she braids her hair, so she asked the Prophet (may Allah's peace and blessings be upon him) how to wash from major ritual impurity: must she undo her hair so that water reaches her scalp? The Prophet (may Allah's peace and blessings be upon him) said that it is unnecessary for her to do so. Rather, it would be sufficient to pour three handfuls of water on her head and assume that the water reaches the roots of the hair, whether the water does reach the scalp or not, because if that was necessary, the braids would have to be undone. The Hadīth, however, does not suggest that. Three handfuls are not necessarily the exact number required; what is required is to let the water reach the roots of the hair. If it reaches them with one handful, then pouring three handfuls is an act of Sunnah. If it does not reach them even after three handfuls, then pouring more handfuls is obligatory until one believes that the hair is most probably soaked. To summarize, the Prophet instructed Umm Salamah not to undo her hair to wash from sex or menstruation. Pouring three handfuls of water on her head then pouring water over her whole body purifies her from major ritual impurity.