Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: I passed by the Messenger of Allah (peace and blessings be upon him) and my lower garment (izar) was hanging down. He said: "O Abdullah, raise your lower garment." I raised it, and he said: "Higher." So I raised it higher, and after that I always kept striving (to keep it so). Some people asked: Up to where? He said: To the middle of the shins.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3316