It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would let his lower garment hang down in front and keep it raised at the back."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4944
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ابن سعد] عن يزيد بن أبي حبيب مرسلا. الصحيحة ١٢٣٨: ابن سعد، هب - ابن عباس.