Sunan an-Nasa'i 14
It was narrated that Anas bin Malik said:
"A time limit was set for us, by the Messenger of Allah (ﷺ), regarding trimming the mustache, clipping the nails and plucking the pubes; we were not to leave that for more than forty days," on one occasion he said: "Forty nights."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا جَعْفَرٌ، - هُوَ ابْنُ سُلَيْمَانَ - عَنْ أَبِي عِمْرَانَ الْجَوْنِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ وَقَّتَ لَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم فِي قَصِّ الشَّارِبِ وَتَقْلِيمِ الأَظْفَارِ وَحَلْقِ الْعَانَةِ وَنَتْفِ الإِبْطِ أَنْ لاَ نَتْرُكَ أَكْثَرَ مِنْ أَرْبَعِينَ يَوْمًا ‏.‏ وَقَالَ مَرَّةً أُخْرَى أَرْبَعِينَ لَيْلَةً ‏.‏