1705/1 - Mu‘ādh ibn Anas al-Juhani (may Allah be pleased with him) reported that the Prophet (may Allah’s peace and blessings be upon him) forbade Habwah (a posture of sitting) on Friday while the Imām is delivering the sermon. [Narrated by Abu Dāwūd and Al-Tirmidhi, who both classified it as Hasan (sound)]
Habwah: a posture of sitting in which the legs are contracted towards the belly, and the back is bent forwards and supported by putting part of the clothing or the arms across the knees.
1) The prohibition on the posture of Habwah is limited to the time of the Friday sermon only.
2) It urges us to listen attentively to the sermon and focus on nothing else.
The purposes behind prohibiting the posture of Habwah include the following:
If one is wearing a single garment, his private parts may get uncovered as he moves while being in this posture. The posture of Habwah causes sleepiness and thus one will be distracted from listening to the sermon and his ablution may get broken, and Allah knows best.