Jami At Tirmidhi
294
Sahih+2

Ibn Umar narrated:"When the Prophet would sit during the Salat, he would place his right hand on his knee, and raise his finger, the one that is next to the [right] thumb, supplicating with it, and his left hand was spread flat on his left knee

SahihAhmad Muhammad Shakir
SahihAl-Albani