I said, "Messenger of Allah, a child of the Ansar who has not yet reached the age of puberty has died, a sparrow among the sparrows of Paradise."He replied, "Or otherwise, Ayesha. Allah created Paradise and created for it its people, and He created the Fire and created for it its people, while they were still in the loins of their fathers."
حدثنا سفيان حدثنا طلحة بن يحيى عن عائشة بنت طلحة
قلت يا رسول الله إن صبيا للأنصار لم يبلغ السن عصفور من عصافير الجنة قال أو غير ذلك يا عائشة خلق الله الجنة وخلق لها أهلا وخلق النار وخلق لها أهلا وهم في أصلاب آبائهم
Its chain of transmission is sahih. It is also found in Muslim 4/2050 no. 2660 in the chapter on divine decree, Abu Dawud 4/226 no. 4713 in the chapter on the sunnah, Nasai 4/57 no. 1947 in the chapter on funerals, and Ibn Majah 1/32 no. 82 in the introduction.