What is the ruling on insulting one's mother?

Cursing one's mother is one of the gravest sins and worst traits, as stated in the Hadith (definition): "Indeed, one of the greatest major sins is for a man to curse his parents." The Prophet, peace and blessings be upon him, explained this by saying that a man curses someone else's father, who then curses his father, and curses his mother. If this is the case for someone who causes their parents to be cursed, then what about someone who curses them directly? Allah has forbidden even saying "uff" to parents, prohibited scolding them, and commanded speaking to them with honorable words.