It is narrated from Yahya bin Sa'id (may Allah have mercy on him) that when al-Qasim bin Muhammad (may Allah have mercy on him) sat in the tashahhud, he would keep his right foot upright, lay his left foot flat, sit on his left hip, and not sit on his feet. And he would say: I saw Abdullah bin Abdullah bin Umar (may Allah be pleased with them both), and he narrated to me that his father used to do the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2778