Tawus reported: We asked Ibn Abbas (RA) about sitting on one's buttocks (in prayer). (ala alqadmein) 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) .
Hadith ReferenceSahih Muslim / The Book of Mosques and Places of Prayer / Hadith: 536