Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5946
'Urwa reported: 'Aisha said to me: Your fathers (Zubair and Abu Bakr) were amongst those about whom (it has been revealed):" Those who responded to the call of Allah and His Messenger after the misfortune had fallen upon them."