سنن ابن ماجه

It was narrated from ‘Amr bin Shu’aib, from his father, that his grandfather said: “The Messenger of Allah (ﷺ) forbade Ihtiba’ (sitting with one's thighs gathered up against the stomach while wrapping one's arm or garment around them) on Fridays, meaning, when the Imam is delivering the sermon.”