Sahih Muslim
The Book Pertaining to Clothes and Decoration (Kitab Al-Libas wal-Zinah)
Book Number: 024 / Hadith Number: 5200
Ibn 'Umar reported: I happened to pass before Allah's Messenger (may peace be upon bin) with my lower garment trailing (upon the ground). He said: 'Abdullah, tug up your lower garment,, I tugged it up, and he again said: Tug it still further, and I tugged it still further and I went on tugging it afterward, whereupon some of the people said: To what extent? Thereupon he said: To the middle of the shanks.