How sound is the statement attributed to Omar ibn al-Khattab, may Allah be pleased with him, that he wept upon hearing the sound of thunder, saying, "This is the sound of His mercy; how, then, will be the sound of His torment?"

The story mentioned is not about Umar ibn al-Khattab, but rather about Umar ibn Abd al-Aziz with Sulayman ibn Abd al-Malik. It was narrated by Abu Bakr al-Dinawari al-Maliki in "Al-Mujalasah wa Jawahi al-Ilm" with a chain of transmission that is not accused, even if there is slight discussion regarding the memory of Ata' or Ishaq's hearing from Jarir. It has other corroborating narrations mentioned by Abu al-Shaykh, Ibn Abi al-Dunya, and Abu Nu'aym, which collectively render the story authentic. Al-Dhahabi also mentioned it in "Siyar A'lam al-Nubala'" and "Tarikh al-Islam."