Yahya related to me from Malik from Yahya ibn Said that al-Qasimibn Muhammad showed them how to sit in the tashahhud, and he kept hisright foot vertical and laid his left foot down, and sat on his lefthaunch not on his foot. Then he said, "Abdullah ibn Abdullah ibn Umarsaw me doing this and related to me that his father used to do thesame thing."
App reference: Book 3, Hadith 55
USC-MSA web (English) reference: Book 3, Hadith 55
Arabic reference: Book 3, Hadith 202