He traced it back to the Prophet ﷺ, who said, "The Hour will not come until you fight a people whose sandals are made of hair, and the Hour will not come until you fight a people with small eyes."
حدثنا أبو بكر بن أبي شيبة حدثنا سفيان بن عيينة عن الزهري عن سعيد بن المسيب
يبلغ به النبي ﷺ قال لا تقوم الساعة حتى تقاتلوا قوما نعالهم الشعر ولا تقوم الساعة حتى تقاتلوا قوما صغار الأعين
Bukhari, Jihad, chapter on fighting those who wear sandals of hair, hadith no. 2929, and Muslim, Fitan, chapter "The Hour will not come until a man passes by another man's grave..." etc., hadith no. 57/2909, from Ibn Abu Shayba, from the hadith of Sufyan, with this wording.