How can the obligation of wiping the entire head during ablution – based on the understanding of the Quranic verse "فَامْسَحُوا بِرُءُوسِكُمْ" (wipe over your heads) – be reconciled with the authentic hadith which mentions that the Messenger of Allah, peace and blessings be upon him, used to wipe over his forelock (a quarter of the head)?

Wiping the head during ablution (wudu') is obligatory, and scholars have differed on the sufficient amount to wipe. The hadith (definition) referred to by the questioner indicates that wiping the entire head is not obligatory according to those who hold that view, while some scholars interpreted it as applicable in cases of necessity. What I am convinced of regarding the obligation to wipe the entire head during ablution is closer to piety (wara') and caution, in order to avoid scholarly differences.