Imam Bukhari : 5438

حَدَّثَنَا الْمَكِّيُّ بْنُ إِبْرَاهِيمَ عَنْ حَنْظَلَةَ عَنْ نَافِعٍ ح قَالَ أَصْحَابُنَا عَنْ الْمَكِّيِّ عَنْ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَاعَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ مِنْ الْفِطْرَةِ قَصُّ الشَّارِبِ

"Narrated to us Al-Makki bin Ibrahim from Hanzalah from Nafi' and our companions said: from Al-Makki from Ibn Umar, may Allah be pleased with them, from the Prophet, peace and blessings be upon him, he said: 'Part of the natural disposition is trimming the mustache.'