Musannaf Ibn Abi Shayba 22:357

Abu al-Ahwas narrated to us, from Abu Ishaq, from Ikrimah, from Ibn Abbas, who said: Abu Bakr said, "O Messenger of Allah, what has made you gray?" He said, "Surah Hud, al-Waqi'ah, al-Mursalat, 'Amma yatasa'alun (an-Naba'), and 'Idha ash-shamsu kuwwirat (at-Takwir) have made me gray."