Al-Hasan ibn Mahbub narrated from Ali ibn Ri'ab from Zurarah who said:
I said to Abu Ja'far عليه السلام: “Is there a known limit between Dhuhr and Asr?” Imam عليه السلام said: "No."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 273