Musannaf Ibn Abi Shayba 3:4410

Ibn ‘Ulayyah narrated to us, from Ma’mar, from a man who said: Sa’id ibn al-Musayyib saw a man while he was playing with his beard during prayer. He said: "If the heart of this one had attained khushu’ (humility/reverence), his limbs would have attained khushu’."