"I strictly condemn the derogatory remarks made by a DAP member, totally unbecoming. We may differ in our political opinion but being abusive is uncalled for.
"This type of action incites disharmony as after all, we are all Malaysians," he said in a statement published on his official Facebook page early Saturday.
Najib is currently away on a three-day official visit to Kazakhstan.
"At the same time, I call upon all Umno members not to overreact by doing things against the law or deemed to be irresponsible.
"Any reaction should be within the bounds of the law and we must strive to continue the good image of our party. Jaga nama baik parti kita (please honour our party's good name)," he said.
The "Umno celaka" remark uttered by Penang DAP's Seri Delima representative R.S.N. Rayer in the state assembly earlier in the week has sparked anger among Umno members, who have staged a series of protests in Penang, Kuala Lumpur and Kuantan over Rayer's refusal to apologise.