Musannaf Ibn Abi Shayba 18:217

Abu Bakr narrated to us, he said: Muhammad bin Bakr narrated to us, from Ibn Jurayj, he said: It was narrated to me from Sa’id bin al-Musayyib that he said regarding the human jaw: That one should fold his thumb, then place its bone, and open his mouth, then place it between his two jaws. Whatever is lacking from the opening of his mouth compared to the lower bone of his thumb, it is according to its calculation.