Musannaf Ibn Abi Shayba 3:2448

Abu Khalid al-Ahmar narrated to us, from Ibn ‘Ajlan, from Yazid, from Abu Khusayfah, from Sa’id ibn al-Musayyib, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you is in the mosque, let him not interlace his fingers."