Musnad Ahmad 30:519

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from 'Ata' ibn Yazid al-Laythi, from Abu Hurayrah, who said: The Messenger of Allah, may Allah bless him and grant him peace, was asked about the children of the polytheists. He said: "Allah knows best what they would have been doing."