"The bomber was about 17 years old", he added later.
A bomber struck inside a mosque packed with worshippers during morning prayers in the town of Mubi.
The latest attack is the highest loss of life since 56 people were killed in the same state last December, when two schoolgirl suicide bombers killed 56 people and wounded dozens more in a crowded market.
"Already, the injured have been rushed to hospitals".
Boko Haram has been blamed for more than 20,000 deaths during its almost decade-old insurgency, which has spilled over into neighbouring countries and displaced millions of people, creating a vast humanitarian crisis. However, a reporter on the scene said the death toll was no more than 27, all men and boys.
There was no immediate claim of responsibility for the attack, but terrorist group Boko Haram has been behind many such attacks in the past.
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In a statement on Tuesday, Nigeria's president said the attack was "very cruel" and gave assurances his government would do "everything required" to secure the state from Boko Haram.
Boko Haram has waged an insurgency in northeastern Nigeria since 2009 in its attempt to create an Islamic state in the region.
Yet in recent months, Boko Haram activity has been concentrated around Madagali, in the far north of Adamawa near the border with neighbouring Borno state.
But it has been peaceful since the military and the civilian militia ousted them from the town, which is a commercial hub and home to the Adamawa State University.
Ryan Cummings, from security analysts Signal Risk, said the attack suggests Boko Haram "has an active operational presence in Adamawa" and retained the capacity to hit hard.