Nigeria's women's affairs minister submitted her resignation in a letter to the president on Saturday, becoming the second cabinet member to step down this month and adding that she would leave the ruling party.
Aisha Alhassan, a relatively junior cabinet member, resigned a little over two weeks after Kemi Adeosun, who had overseen finance, stepped down over allegations she had used a forged certificate to avoid mandatory national service.
The resignations are the first in Buhari's cabinet since he came to power in 2015, and come in the run-up to early 2019 elections, typically a time where fluid political allegiances in Nigeria change.
In Alhassan's letter to Buhari, seen by Reuters, the minister said she was resigning and leaving the leader's ruling All Progressives Congress (APC) party because she had been barred from running for governor of eastern Taraba state.
"I received with shock and dismay that I was disqualified and not cleared to contest for 2019 governorship election by the National Working Committee of the party (APC)," her letter said.
"No reason was given for my disqualification."
A spokesman for Buhari said he could not confirm the letter.
Last year, Alhassan told Reuters that Buhari had said he would only seek one term. He is currently campaigning for re-election.
Alhassan said at the time she would resign if Buhari sought a second term and that she would support former vice president Atiku Abubakar if he decided to run.
Atiku later quit the APC and is now running as a presidential candidate for the opposition People's Democratic Party.