It was the practice of the Prophet (peace and blessings be upon him):
"He (peace and blessings be upon him) would sit on his buttocks with his knees drawn up."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4914
Hadith Gradingالألبانی:حسن
Hadith Takhrij[طب] عن إياس بن ثعلبة. الصحيحة ٢١٢٤.