Shehu was reacting to Junaidu Mohammed's comment that Buhari does not acknowledge mistakes.
Shehu described Junaidu Mohammed, a former member of the House of Representatives and a fierce critic of the President, as someone suffering from "pathological cynicism."
The presidential spokesman said this in a statement, in reaction to Mohammed's comment that Buhari does not acknowledge mistakes.
On the contrary, Shehu said taking responsibility for mistakes is one of the "finest components" of integrity, which Buhari has demonstrated in words and action.
He said: "It is obvious that Dr. Junaidu appears to be infected by pathological cynicism and these kinds of critics are toxic to national progress because they are only primarily interested in destroying every positive effort and achievements of the government.
"We advise Nigerians to be wary of prejudiced and biased critics like Dr. Junaidu Mohammed who are more interested in personal publicity than respecting facts and figures about their claims.
"If Junaidu Mohammed wants to be taken seriously, he should demonstrate the duty of speaking truthfully about those he frequently attacks in his interviews."
According to him, Mohammed had attacked past presidents, but not Sani Abacha.
"It must be noted that President Buhari was not the only target of Junaidu’s personal vituperations, as former Presidents Shehu Shagari, Ibrahim Babangida, Olusegun Obasanjo,Umaru Yar’adua and Goodluck Jonathan, had all been victims of his jaundiced and malicious attacks," Shehu said.
"When it came to the late Sani Abacha, he tried it, but once he saw the prospect of elimination, he retreated.
"A cursory library search will reveal that this man has nurtured the habit of insulting every leader this country has produced in his time. Like a narcissist, he is just interested in grabbing headlines. Without publicity, Dr. Junaidu would probably give up on life."
He added that Mohammed has been using his "political retirement life" to insult every Nigerian president just for the sake of publicity.