Musnad Ahmad 30:3590

Uthman ibn Umar narrated to us, Ibn Abi Dhi'b informed us, from al-Zuhri, from 'Ata' ibn Yazid, from Abu Hurayrah, that the Messenger of Allah, peace and blessings of Allah be upon him, was asked about the children of the polytheists. He said: "Allah is most knowing of what they would have been doing."