"As people converged on Massalacin Madina at Anguwan Shuwa to observe Subh (morning prayer), an unknown man rammed himself into the mosque with IEDs [improvised explosive device], killing many people", a resident told the Premium Times.
Security analysts said Tuesday's bombing again underlined the threat posed by Boko Haram, despite an overall decline in deaths from attacks by the group previous year.
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.
He has also directed the Inspector-General of Police, Ibrahim Idris, to deploy more police personnel in the area to prevent further attacks by armed bandits in Zamfara State.
Abubakar Othman, a police spokesman in Adamawa state, said the death toll from Tuesday's attack in the town of Mubi is 50 people, but "there could be more as those seriously injured could add to the figure".
President Buhari prayed that God would console the bereaved families in the Adamawa and Zamfara incidents, heal the injured, and grant rest to the souls of the departed.
Quake of magnitude 6.4 strikes near New Caledonia
The United States Geological Survey said the quake struck 51 miles east of the Loyalty Islands at 9.43am local time (10.43pm GMT).
"The roof was blown off. People near the mosque said the prayer was mid-way when the bomber, who was obviously in the congregation, detonated his explosives".
The latest Global Terrorism Index, published last week, said that deaths attributed to Boko Haram in 2016 fell by 80 percent.
He said, "This wanton and deliberate targeting of civilians must end".
Mubi is in Adamawa state where Boko Haram held territory in 2014. During the occupation, the town's name was changed temporarily to Madinatul Islam, or "City of Islam" in Arabic.
In recent months, Boko Haram activity has been concentrated around Madagali, in the far north of Adamawa near the border with Borno.
No-one has said they were behind the bombing but the Islamist militants Boko Haram typically target crowded places in northern Nigeria.
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