Sunan an-Nasa'i, Book 13, Hadith 91 (In-Book) ⓘ

It was narrated from Ibn 'Umar that: When the Messenger of Allah sat during the prayer, he put his hands on his knees and raised the finger that is next to the thumb, and supplicates with it, and his left hand was on his knee laid on it.

أَخْبَرَنَا مُحَمَّدُ بْنُ رَافِعٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ إِذَا جَلَسَ فِي الصَّلاَةِ وَضَعَ يَدَيْهِ عَلَى رُكْبَتَيْهِ وَرَفَعَ أُصْبُعَهُ الَّتِي تَلِي الإِبْهَامَ فَدَعَا بِهَا وَيَدَهُ الْيُسْرَى عَلَى رُكْبَتِهِ بَاسِطُهَا عَلَيْهَا ‏.‏