Bulugh al-Maram, Book 1, Hadith 39 (In-Book)

Narrated ‘Ali (rad) regarding the description of the ablution of the Prophet: He (the Prophet) wiped his head (with water) only once. [Reported by Abu Da’ud, An-Nasa’i, AtTirmidhi with a Sahih Sanad (authentic chain of narrators). At-Tirmidhi said, “It’s the most Hadith on this subject”].

وَعَنْ عَلِيٍّ ‏- رضى الله عنه ‏- ‏-فِي صِفَةِ وُضُوءِ اَلنَّبِيِّ صَلَّى اَللَّهُ عَلَيْهِ وَسَلَّمَ‏- قَالَ: { وَمَسَحَ بِرَأْسِهِ وَاحِدَةً.‏ } أَخْرَجَهُ أَبُو دَاوُد َ (44)‏.‏ ‏3 ‏- صحيح.‏ رواه أبو داود (111)‏.‏

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also referenced under: Book 1, Hadith 38 (English)