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 664
It was narrated that 'Ali said:
"A man came to the Prophet and said: 'I bathed to cleanse myself from sexual impurity, and I prayed Fajr, then I noticed a spot the size of a fingernail that the water did not reach.' The Messenger of Allah said: 'If you had wiped it that would have been sufficient for you.'"
حَدَّثَنَا سُوَيْدُ بْنُ سَعِيدٍ، حَدَّثَنَا أَبُو الأَحْوَصِ، عَنْ مُحَمَّدِ بْنِ عُبَيْدِ اللَّهِ، عَنِ الْحَسَنِ بْنِ سَعْدٍ، عَنْ أَبِيهِ، عَنْ عَلِيٍّ، قَالَ جَاءَ رَجُلٌ إِلَى النَّبِيِّ ـ صلى الله عليه وسلم ـ فَقَالَ إِنِّي اغْتَسَلْتُ مِنَ الْجَنَابَةِ وَصَلَّيْتُ الْفَجْرَ ثُمَّ أَصْبَحْتُ فَرَأَيْتُ قَدْرَ مَوْضِعِ الظُّفْرِ لَمْ يُصِبْهُ الْمَاءُ ‏.‏ فَقَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ‏ "‏ لَوْ كُنْتَ مَسَحْتَ عَلَيْهِ بِيَدِكَ أَجْزَأَكَ ‏"‏ ‏.‏
Grade: Da'if (Darussalam)
Reference : Sunan Ibn Majah 664
In-book reference : Book 1, Hadith 398
English translation : Vol. 1, Book 1, Hadith 664