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 c

'Ali b. 'Abual-Rahman al-Mu'awi reported:

'Abdullah b. Umar saw me playing with pebbles during prayer. After finishing the prayer he forbade me (to do it) and said: Do as the Messenger of Allah (ﷺ) used to do. I said: How did Allah's Messenger (ﷺ) do? He said that he (the Messenger of Allah) sat at tashahhud, placed his right palm on the right thigh and closed all his fingers and pointed with the help of finger next to the thumb, and placed his left palm on his left thigh.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنْ مُسْلِمِ بْنِ أَبِي مَرْيَمَ، عَنْ عَلِيِّ بْنِ عَبْدِ الرَّحْمَنِ الْمُعَاوِيِّ، أَنَّهُ قَالَ رَآنِي عَبْدُ اللَّهِ بْنُ عُمَرَ وَأَنَا أَعْبَثُ بِالْحَصَى فِي الصَّلاَةِ فَلَمَّا انْصَرَفَ نَهَانِي فَقَالَ اصْنَعْ كَمَا كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصْنَعُ ‏.‏ فَقُلْتُ وَكَيْفَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصْنَعُ قَالَ كَانَ إِذَا جَلَسَ فِي الصَّلاَةِ وَضَعَ كَفَّهُ الْيُمْنَى عَلَى فَخِذِهِ الْيُمْنَى وَقَبَضَ أَصَابِعَهُ كُلَّهَا وَأَشَارَ بِإِصْبَعِهِ الَّتِي تَلِي الإِبْهَامَ وَوَضَعَ كَفَّهُ الْيُسْرَى عَلَى فَخِذِهِ الْيُسْرَى ‏.‏
Reference : Sahih Muslim 580c
In-book reference : Book 5, Hadith 149
USC-MSA web (English) reference : Book 4, Hadith 1204
  (deprecated numbering scheme)