From Sahih Muslim (Hadith 5462):
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
Reference: Muslim 5462