Shababah bin Sawwar (may Allah have mercy on him) relates in this narration that when the Prophet (peace and blessings be upon him) sat in the first two rak'ahs, he would sit on his left foot and keep his right foot upright; and when he sat in the final rak'ah, he would sit on his buttocks, bring the inner part (sole) of his left foot close to the back of the knee of his right thigh, and keep his right foot upright.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2772