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 " .
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.'" Translation Not Available
Book Ref: Sunan Ibn Majah Book 1 Hadith 664
Web Ref: Sunan Ibn Majah Vol 1 Book 1 Hadith 664