Al-Kāfi - Volume 6 > The Under arms
Hadith #1

عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ النَّوْفَلِيِّ عَنِ السَّكُونِيِّ عَنْ أَبِي عَبْدِ اللَّهِ ( عليه السلام ) قَالَ قَالَ رَسُولُ اللَّهِ ( صلى الله عليه وآله ) لَا يُطَوِّلَنَّ أَحَدُكُمْ شَعْرَ إِبْطِهِ فَإِنَّ الشَّيْطَانَ يَتَّخِذُهُ مَخْبَأً لِيَسْتَتِرَ بِهِ .

1. Ali ibn Ibrahim has narrated from his father from al-Nawfaliy from al-Sakuniy who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said that the Messenger of Allah, O Allah, grant compensation to Muhammad and his family worthy of their services to Your cause, has said, ’No one of you must allow the hairs of his underarms to grow long; Satan makes it his hiding place and cover.”’