Musannaf Ibn Abi Shayba 33:393

Ibn Numayr narrated to us, from Ubayd Allah ibn Umar, from Sa’id ibn Abi Sa’id, from Amr ibn Abi Bakr, from his father, from Ka’b al-Ahbar, who said: I find in the Book of Allah: There is no believing servant who goes to the mosque and returns—not going or returning except to learn good or teach it, or to remember Allah or be reminded of Him—except that his likeness in the Book of Allah is like the likeness of the mujahid in the way of Allah. And Allah the Exalted knows best.