The bomb exploded at a branch of the First Bank of Nigeria in Azare in Bauchi state, an area that has suffered previous attacks carried out by the Islamist insurgent group Boko Haram, the source, who asked not to be named, told Reuters.
Local media reported that the victims had been lining up to use an ATM machine at the bank.
On 22 October, a bomb exploded at a bus station in Azare, killing at least five people and wounding 12.
There has been a spate of attacks by suspected Boko Haram militants in the northeast of Nigeria in the last few weeks, raising doubts about a government announcement last month that it had reached a ceasefire with the Islamist group.
Boko Haram had announced the establishment of an Islamic territory in part of the northeast, following its capture of a string of towns. It has killed thousands in a five-year-old insurgency in Africa’s biggest economy and leading oil producer.