(1) The Book of Purification and its Sunnah
كتاب الطهارة وسننها
(8) Chapter: The Fitrah (natural inclination of man)
(8) باب الْفِطْرَةِ
Sunan Ibn Majah 295 
It was narrated that Anas bin Malik said:
"We were given a time limit with regard to trimming the mustache, shaving the pubic hairs, plucking the armpit hairs and clipping the nails. We were not to leave that for more than forty days."
حَدَّثَنَا بِشْرُ بْنُ هِلاَلٍ الصَّوَّافُ، حَدَّثَنَا جَعْفَرُ بْنُ سُلَيْمَانَ، عَنْ أَبِي عِمْرَانَ الْجَوْنِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ وُقِّتَ لَنَا فِي قَصِّ الشَّارِبِ وَحَلْقِ الْعَانَةِ وَنَتْفِ الإِبْطِ وَتَقْلِيمِ الأَظْفَارِ أَنْ لاَ نَتْرُكَ أَكْثَرَ مِنْ أَرْبَعِينَ لَيْلَةً ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan Ibn Majah 295
In-book reference : Book 1, Hadith 29
English translation : Vol. 1, Book 1, Hadith 295