From Sahih Muslim (Hadith 1198):
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 (ﷺ)
Reference: Muslim 1198