Al-wudu’ is either obligatory or recommended.
a- There are three things for which al-wudu is obligatory.
1- As-Salah (Prayers)
Allah (exalted be He) said: “O you who believe, when you intend to offer As-Salah (prayers), wash your faces and your hands up to the elbows, rub your heads, and wash your feet up to (and including) the ankles.” (Al-Ma’idan:6).
2- Circumambulation of Ka’bah.
This is because the Prophet ﷺ said to a menstruating woman: “Do not perform Tawaf until you have taken a purificatory bath.” [ narrated by Al-Bukhari.]
3- Touching the Noble Qur’an:
Allah the most Exalted says: “None touch it (The Noble Qur’an) except the purified”(Waqiah:79)
The Prophet ﷺ said: “My nation will come forth on the day of judgment like ‘Ghurrah Muhajjaleen’ (horses with white forelocks and white feet)[ Al-Ghurru: is the white patch on the forehead of the horse. Muhajjaleen: are horses with white feet. At-Tahjeel: White patches on their limbs. This refers to the light on the Day of Judgment that will be on the parts of the body that used to be washed during al-wudu’.] as a result of making Al-wudu’. So any of you (who is) capable of lengthening his ghurrah (places of light) should do so.” [ agreed upon.]