مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2833
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
Tawoos said: We asked Ibn ‘Abbas (رضي الله عنه) about sitting on the heels with the feet upright, and he said: This is the Sunnah. We said: We think it is difficult for a man. Ibn ‘Abbas (رضي الله عنه) said: It is the Sunnah of your Prophet (ﷺ).
Musnad Ahmad 2853Its isnad is saheeh, Muslim (536)]