Musannaf Ibn Abi Shayba 3:5198

Hafs narrated to us, from Hujayyah, from Anas, who said: Umar saw me while I was praying towards a grave, and he began saying: "O Anas, the grave! The grave!" I began raising my head, looking at the moon. So they said: "He is only saying: 'The grave.'"