The Book of Purification
1/15. Chapter:
Trimming The Mustache And Letting The Beard Grow
Sunan an-Nasai
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، - هُوَ ابْنُ سَعِيدٍ - عَنْ عُبَيْدِ اللَّهِ، أَخْبَرَنِي نَافِعٌ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " أَحْفُوا الشَّوَارِبَ وَأَعْفُوا اللِّحَى " .
It was narrated from Ibn 'Umar that the Prophet (ﷺ) said: "Trim the mustache and let the beard grow."
Reference : Sunan an-Nasa'i 15In-book reference : Book 1, Hadith 15English translation : Vol. 1, Book 1, Hadith 15