Hadith: If a Muslim or - a believer - performs ablution, when he washes his face, every sin he has committed with his eyes is washed away with the water, or with the last drop of water

عن أبي هريرة رضي الله عنه مرفوعاً: «إذا توضَّأ العبدُ المسلم، أو المؤمن فغسل وَجهَهُ خرج مِنْ وَجْهِهِ كُلُّ خَطِيئَةٍ نظر إليها بِعَينَيهِ مع الماء، أو مع آخر قَطْرِ الماء، فإذا غسل يديه خرج من يديه كل خطيئة كان بَطَشَتْهَا يداه مع الماء، أو مع آخِرِ قطر الماء، فإذا غسل رجليه خرجت كل خطيئة مَشَتْهَا رِجْلَاه مع الماء أو مع آخر قطر الماء حتى يخرج نَقِيًا من الذنوب».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "If a Muslim or - a believer - performs ablution, when he washes his face, every sin he has committed with his eyes is washed away with the water, or with the last drop of water; when he washes his hands, every sin he committed is washed away from his hands with the water, or with the last drop of water; and when he washes his feet, every sin that his feet walked towards is washed away with the water, or with the last drop of water, until he comes out cleansed of all sins."
[Authentic hadith] - [Narrated by Muslim]

Explanation

Ablution cleanses four body parts: the face, the hands, the head, and the feet. This cleansing is both physical and spiritual. Physical cleansing is obvious because the person washes his face, his hands/arms, his feet, and wipes his head. The head could have been washed like the other parts but Allah lightened its cleansing to only wiping (with a wet hand). This is because the head carries the hair and it is the uppermost organ on a person's body. If one were to wash his head, especially if he has a lot of hair, this would be difficult, especially in cold weather. So, out of Allah's mercy, He reduced the obligation of cleansing the head to wiping instead of washing. When a person makes ablution, no doubt he physically cleanses those body parts, which indicates the perfectness of Islamic rituals as they oblige Muslims to cleanse these parts of their bodies which are mostly visible (and thus exposed to dust and the like). As to spiritual cleansing, it should be the intention of the Muslim in performing ablution to cleanse himself from sins. If one washes his face, all the sins that he saw with his eyes will be washed away. Allah mentioned the eyes - and Allah knows best - just as an example, otherwise the nose and the mouth may also sin; the person may speak unlawful words or smell something that he has no right to smell. However, the eyes were mentioned because most of the of sins come from looking at things a person is not permitted to look at.