كتاب الطهارة وسننها
1  
The Book of Purification and its Sunnah
(138)
Chapter: One who bathed to cleanse himself from sexual impurity and there remained a spot on his body that was not touched by water: what should he do?
(138)
باب مَنِ اغْتَسَلَ مِنَ الْجَنَابَةِ فَبَقِيَ مِنْ جَسَدِهِ لُمْعَةً لَمْ يُصِبْهَا الْمَاءُ كَيْفَ يَصْنَعُ
Sunan Ibn Majah 663
It was narrated from Ibn 'Abbas:
"The Prophet bathed to cleanse himself from sexual impurity, then he saw a spot that the water did not reach." Then he motioned with the hair hanging over his shoulders and squeezed (the water from it) over that spot." (Da'if)In his narration, Ishaq said: "So he wrung his hair over it."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَإِسْحَاقُ بْنُ مَنْصُورٍ، قَالاَ حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، أَنْبَأَنَا مُسْتَلِمُ بْنُ سَعِيدٍ، عَنْ أَبِي عَلِيٍّ الرَّحَبِيِّ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ اغْتَسَلَ مِنْ جَنَابَةٍ فَرَأَى لُمْعَةً لَمْ يُصِبْهَا الْمَاءُ فَقَالَ بِجُمَّتِهِ فَبَلَّهَا عَلَيْهَا ‏.‏ قَالَ إِسْحَاقُ فِي حَدِيثِهِ فَعَصَرَ شَعْرَهُ عَلَيْهَا ‏.‏
Reference : Sunan Ibn Majah 663
In-book reference : Book 1, Hadith 397
English translation : Vol. 1, Book 1, Hadith 663