Muwatta Malik #201
Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّ الْقَاسِمَ بْنَ مُحَمَّدٍ، أَرَاهُمُ الْجُلُوسَ فِي التَّشَهُّدِ فَنَصَبَ رِجْلَهُ الْيُمْنَى وَثَنَى رِجْلَهُ الْيُسْرَى وَجَلَسَ عَلَى وَرِكِهِ الأَيْسَرِ وَلَمْ يَجْلِسْ عَلَى قَدَمِهِ ثُمَّ قَالَ أَرَانِي هَذَا عَبْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُمَرَ وَحَدَّثَنِي أَنَّ أَبَاهُ كَانَ يَفْعَلُ ذَلِكَ .
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.