Sahl bin Mu'adh bin Anas al-Juhani narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) forbade "sitting with the knees drawn up (ihtiba') on Friday during the sermon."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5912