مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2835
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Tawoos said: I saw Ibn ‘Abbas (رضي الله عنه) sitting on the balls of his feet (when his feet were spread out) and I said: The people claim that this is difficult. He said: It is the Sunnah of your Prophet (ﷺ).
Musnad Ahmad 2855A saheeh hadeeth]