Is it permissible for a Muslim's ablution (wudu') to wipe the forelock, while maintaining his prayer?

Abusing a Muslim is forbidden and an act of wickedness (fisq). If this abuse does not recur from you, it is considered a minor sin that can be expiated by performing ablution (wudu') — provided it is a complete ablution like that of the Prophet, peace and blessings be upon him — or by performing acts that expiate sins, such as fasting on the Day of 'Arafah. As for major sins, they are only expiated by sincere repentance. Persistence in minor sins transforms them into major ones. The tongue plays a dangerous role if it is not guarded, for it is a cause for people entering Hellfire.