From him, from Al-Abbas, from Hammad, from Hariz, from Abu Abdullah عليه السلام, who said:
Imam عليه السلام said: "A man should not look at the private parts of his brother."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 397