What is the ruling on washing the head in Wudu’ instead of wiping it? Does it invalidate Wudu’? What is the evidence for that? And why did Ibn ‘Umar (may Allah be pleased with them both) wipe part of his head, contrary to the action of the Prophet (peace be upon him)?

Washing the head instead of wiping it during ablution is disliked due to the excessiveness and hardship involved, but it does not invalidate the ablution according to the majority of scholars, because it is wiping and more. As for the issue of wiping part of the head, it is a point of contention among scholars due to their differing interpretations of the particle "bi" (بِ) in His (the Most High) saying: {And wipe your heads (وَامْسَحُوا بِرُؤُوسِكُمْ)}. Those who interpret it as indicating partiality permitted wiping part of the head and used as evidence the Prophet's (peace be upon him) wiping over his forelock and turban. Meanwhile, others held the view that wiping the entire head is obligatory.