Musannaf Ibn Abi Shayba 3:2449

Abu Khalid al-Ahmar narrated to us, from Sa'd ibn Ishaq, from Sa'id ibn Abi Thumama al-Qammah, who said: I met Ka'b while I was at al-Balat, and I had interlaced some of my fingers with others. He struck my hand a severe strike and said, "We were forbidden from interlacing our fingers while in prayer." I said to him, "May Allah have mercy on you, do you see me in prayer?" He replied, "Whoever performs ablution and then heads to the mosque, he is in prayer."