Musnad Ahmad 31:629

Muhammad ibn Mus'ab narrated to us, 'Umarah narrated to us, from Abu Nadrah, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Thunderbolts will increase when the Hour draws near, until a man comes to a people and says: 'Who was struck by a thunderbolt among you this morning?' They will say: 'So-and-so and so-and-so were struck.'"