“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.”
نَهَى رَسُولُ اللَّهِ ﷺ عَنْ الاِحْتِبَاءِ يَوْمَ الْجُمُعَةِ يَعْنِي وَالإِمَامُ يَخْطُبُ