Abu Umar, meaning Uzai, said that this is when you see the sunlight on the ground turn yellow.
حدثنا محمود بن خالد
قال أبو عمرو يعني الأوزاعي وذلك أن ترى ما على الأرض من الشمس صفراء
No references are available.
[Daif] * Walid bin Muslim is a mudallis, he practiced tadlis al-taswiyah, and I did not find an explicit statement of direct transmission.