Musnad Abu Ya'la al-Mawsili 155:280

Abu Musa narrated to us, Yahya bin Sa'id narrated to us, from Muhammad bin 'Amr, from Abu Salamah, from Abu Hurairah: That the Messenger of Allah (ﷺ) was asked about the children of the polytheists. He said: "Allah knows best what they would have been doing."