(13) The Book of Forgetfulness (In Prayer)
كتاب السهو
(30) Chapter: Placement of the forearms
(30) باب مَوْضِعِ الذِّرَاعَيْنِ
Sunan an-Nasa'i 1264 
It was narrated from Wa'il bin Hujr that :
He saw the Prophet (ﷺ) sitting during the prayer. He lay his left foot on the ground and placed his forearms on his thighs, and pointed with his forefinger, supplicating with it.
أَخْبَرَنَا مُحَمَّدُ بْنُ عَلِيِّ بْنِ مَيْمُونٍ الرَّقِّيُّ، قَالَ حَدَّثَنَا مُحَمَّدٌ، - وَهُوَ ابْنُ يُوسُفَ الْفِرْيَابِيُّ - قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ عَاصِمِ بْنِ كُلَيْبٍ، عَنْ أَبِيهِ، عَنْ وَائِلِ بْنِ حُجْرٍ، أَنَّهُ رَأَى النَّبِيَّ صلى الله عليه وسلم جَلَسَ فِي الصَّلاَةِ فَافْتَرَشَ رِجْلَهُ الْيُسْرَى وَوَضَعَ ذِرَاعَيْهِ عَلَى فَخِذَيْهِ وَأَشَارَ بِالسَّبَّابَةِ يَدْعُو بِهَا ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 1264
In-book reference : Book 13, Hadith 86
English translation : Vol. 2, Book 13, Hadith 1265