Musannaf Ibn Abi Shayba 33:1309

Ali bin Hashim narrated to us, from Ibn Abi Layla, from Ibn Abi Mulayka, who said: I saw Abdullah bin Amr while he was weeping, so I looked at him. He said: "Do you wonder that I weep out of fear of Allah? If you do not weep, until one of you says 'Ay, Ay' (an expression of intense grief or sighing), for indeed this moon weeps out of fear of Allah the Exalted."