Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "On the day of Badr the mark of the angels was white turbans which they had let hang down onto their backs, and on the day of Hunayn it was red turbans. The angels did not fight on any day except the day of Badr; otherwise they would only be there as numbers and reinforcement, and would not strike blows themselves."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12033