Sahih Muslim Book - 5, Hadith - 1309

وَحَدَّثَنِى مُحَمَّدُ بْنُ رَافِعٍ وَعَبْدُ بْنُ حُمَيْدٍ قَالَ عَبْدٌ أَخْبَرَنَا وَقَالَ ابْنُ رَافِعٍ حَدَّثَنَا عَبْدُ الرَّزَّاقِ أَخْبَرَنَا مَعْمَرٌ عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ أَنَّ النَّبِىَّ -صلى الله عليه وسلم- كَانَ إِذَا جَلَسَ فِى الصَّلاَةِ وَضَعَ يَدَيْهِ عَلَى رُكْبَتَيْهِ وَرَفَعَ إِصْبَعَهُ الْيُمْنَى الَّتِى تَلِى الإِبْهَامَ فَدَعَا بِهَا وَيَدَهُ الْيُسْرَى عَلَى رُكْبَتِهِ الْيُسْرَى بَاسِطُهَا عَلَيْهَا.

It was narrated from Ibn 'Umar that when the Prophet (s.a.w) sat during the prayer, he would place his hands on his knees, and raise the right finger that is next to the thumb and supplicated with it (i.e. keeping it raised), and his left hand would be spread on his left knee.