Musannaf Ibn Abi Shayba 3:914

Hussain bin Ali narrated to us, from Za’idah, from Mansur, from Ibrahim, who said: We pray Dhuhr when the shadow is three cubits. If a man is in a hurry for a need, he may pray before that. And if something occupies him, he may pray after that. Za’idah said: I said to Mansur, "Is that, meaning that, in the summer?" He said, "Yes."