Sahih Muslim Book 4, Hadith Number 1093.

Chapter : Concerning the command of placing one’s hands on the knees while in Ruku’ and abrogation of al-Tatbiq.

Tawus reported: We asked Ibn Abbas about sitting on one’s buttocks (in prayer). (ala alqad mein)He said: It is sunnah. We said to him: We find it a sort of cruelty to the foot. Ibn ‘Abbas said: It is the sunnah of your Apostle (may peace be upon him).