He traced it back to the Prophet ﷺ that his banner on the day he entered Mecca was white.
حدثنا إسحق بن إبراهيم المروزي وهو ابن راهويه حدثنا يحيى بن آدم حدثنا شريك عن عمار الدهني عن أبي الزبير
يرفعه إلى النبي ﷺ أنه كان لواؤه يوم دخل مكة أبيض
[Hasan]. Tirmidhi transmitted it in the Book of Jihad, Chapter on What Has Been Said about Banners, no. 1679, Nasai, no. 2869, and Ibn Majah, no. 2817, through the hadith of Yahya bin Adam, with this chain. Tirmidhi said, gharib, and it has a hasan corroborating narration in Ibn Majah, no. 2818.