It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he said: Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him) asked the Prophet (peace and blessings be upon him): "(O Messenger of Allah,) what has made you grey?" He (peace and blessings be upon him) said: "Surah Hud, al-Waqi'ah, al-Mursalat, 'Amma yatasa'alun and Idha al-shamsu kuwwirat have made me grey." This hadith is sahih (authentic) on the condition of Imam Bukhari, but the two Shaykhs did not record it.