Musannaf Ibn Abi Shayba 3:893

Yahya ibn Sa'id al-Qattan narrated to us, from Habib ibn Shihab, from his father, who said: I asked Abu Hurayrah about the time of Dhuhr. He said: When the sun has declined from the middle of the day, and the shadow is the length of a sandal strap, then Dhuhr has begun.