كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(21)
Chapter: The description of the sitting during the prayer, and how the hands are to be placed on the thighs
(21)
باب صِفَةِ الْجُلُوسِ فِي الصَّلاَةِ وَكَيْفِيَّةِ وَضْعِ الْيَدَيْنِ عَلَى الْفَخِذَيْنِ ‏
Sahih Muslim 580 b
Another version on the authority of Ibn Umar says:
When the Messenger of Allah (ﷺ) sat for tashahhud, he placed his left hand on his left knee and placed his right hand on his right knee, and he formed a ring like (fifty-three) and pointed with his finger of attestation.
وَحَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، حَدَّثَنَا يُونُسُ بْنُ مُحَمَّدٍ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ أَيُّوبَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا قَعَدَ فِي التَّشَهُّدِ وَضَعَ يَدَهُ الْيُسْرَى عَلَى رُكْبَتِهِ الْيُسْرَى وَوَضَعَ يَدَهُ الْيُمْنَى عَلَى رُكْبَتِهِ الْيُمْنَى وَعَقَدَ ثَلاَثَةً وَخَمْسِينَ وَأَشَارَ بِالسَّبَّابَةِ ‏.‏
Reference : Sahih Muslim 580b
In-book reference : Book 5, Hadith 148
USC-MSA web (English) reference : Book 4, Hadith 1203
  (deprecated numbering scheme)