Mishkat al-Masabih #1411

'Ikrima said that when Ibn ‘Abbas was told that a certain wife of the Prophet had died, he prostrated himself. Someone asked him why he made prostration at such a time, and he replied, “God’s Messenger told us to prostrate ourselves when we saw a sign, and what sign is greater than the departure of the Prophet's wives?” Abu Dawud and Tirmidhi transmitted it.