What is the ruling on looking at unveiled women, whether intentionally or unintentionally?

It is forbidden to look intentionally, based on the Almighty's saying: (Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do). The Prophet, peace and blessings be upon him, likened the eye to a mirror for the heart. If the gaze is lowered, the heart's desire and will are also lowered. The Prophet, peace and blessings be upon him, explained that the eye commits adultery, and its adultery is looking, which indicates the prohibition of looking intentionally.

As for unintentional looking, one is not punished for it if the heart did not intend it. However, if one follows it with another look, one is sinful, based on the Prophet's saying to Ali: (O Ali, do not follow one glance with another, for the first is yours, but the last is not yours).