Abu Dawud Book 2, Hadith Number 1193.

Narated By Ibn Abbas : Ikrimah said: Ibn Abbas was informed that so-and-so, a certain wife of the Prophet (PBUH), had died. He fell down prostrating himself. He was questioned: Why do you prostrate yourself this moment? He said: The Apostle of Allah (PBUH) said: When you see a portent (an accident), prostrate yourselves. And which portent (accident) can be greater than the death of a wife of the Prophet (PBUH).

Complete Translation of Sunan Abu-Dawud, Book 2: Prayer (Kitab Al-Salat): Detailed Rules of Law about the Prayer during Journey