Mishkat al-Masabih 4368
Ibn ‘Umar said:
Part of my lower garment was hanging down when I passed by God’s messenger, so he said, “Raise your lower garment, ‘Abdallah.” I did so and he told mo to raise it higher. I did so, and as I was still trying to find the best place [for it] one of the people asked where it should reach and he replied, “Halfway up the legs.” 
Muslim transmitted it.
عَن ابنِ عمَرِ قَالَ: مَرَرْتُ بِرَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَفِي إِزَارِي اسْتِرْخَاءٌ فَقَالَ: «يَا عَبْدَ اللَّهِ ارْفَعْ إِزَارَكَ» فَرَفَعْتُهُ ثُمَّ قَالَ: «زِدْ» فَزِدْتُ فَمَا زِلْتُ أَتَحَرَّاهَا بَعْدُ فَقَالَ: بَعْضُ الْقَوْمِ: إِلَى أَيْنَ؟ قَالَ: «إِلَى أَنْصَافِ السَّاقَيْنِ» . رَوَاهُ مُسلم