The blast happened in the Mainari area of Konduga, in Borno state, said Ibrahim Liman, from the civilian militia force assisting the military in the fight against Boko Haram.
"The male bomber walked into the mosque at about 5:15 am (0415 GMT) while prayers were on and exploded, killing eight worshippers and injuring five others," he told AFP.
"Seven of the victims died in the mosque while another died on the way to (the Borno state capital) Maiduguri."
Suicide bombings against "soft" civilian targets such as mosques, markets and bus stations are the hallmark of the Boko Haram faction led by Abubakar Shekau.
Many of the bombers used are young women and girls. Liman said the latest attack appeared to be carried out by a man in his early 20s.
Umar Goni, who lives in Konduga, said he was on his way to the mosque when the blast occurred and he helped to rescue victims with members of the civilian volunteer force.
"We pulled out seven dead bodies and six injured worshippers. One of the six injured died on the way to hospital," he added.
The bomber was disguised as a worshipper, he said, adding: "There was no way anybody could have known his mission."
The latest attack comes as Nigeria's government is encouraging people displaced by Boko Haram's Islamist insurgency to return home.
But aid agencies providing food, healthcare, clean water and shelter across the conflict-ravaged region have warned that security has not improved.
Last week, at least six traders were killed when a convoy of lorries under military escort were ambushed in Borno state near the border with Cameroon.
There have also been a number of attacks on military convoys and bases in Borno and neighbouring Yobe state, with undisclosed casualties.
Soldiers and civilians have also been targeted in separate attacks in neighbouring Chad and Niger.