Muwatta Malik #201
Prayer
Yahya related to me from Malik from Yahya ibn Said that al-Qasim
ibn Muhammad showed them how to sit in the tashahhud, and he kept his
right foot vertical and laid his left foot down, and sat on his left
haunch not on his foot. Then he said, "Abdullah ibn Abdullah ibn Umar
saw me doing this and related to me that his father used to do the
same thing."
About Muwatta Malik
This hadith is #201 from Muwatta Malik, in the Book of Prayer. Muwatta Malik contains 1,985 hadiths across 61 books.