(CNN) -- Mayor Thomas Menino, the longest-serving mayor in Boston history, will not seek re-election to a sixth term, a Boston City Hall source said.
Menino, 70, is expected to make the announcement Thursday afternoon at a 4 p.m. news conference with his family at his side.
It's unclear what prompted his decision to not seek re-election, but Menino recently spent eight weeks in a hospital for treatment of a respiratory infection and a blood clot. Menino has cut back on public appearances since his hospitalization.
The source would not say what prompted the mayor's decision.
Menino all but confirmed the decision as he left his house Thursday morning. He told reporters gathered outside that it was "a sad day, but a day that always will come in your career."
"It's a difficult decision when you have something you really loved. ... It's tough to say no, but there's time and a place for everything," Menino said.
Menino was first elected to the mayor's office in 1993
CNN's Lawrence Crook III contributed to this report.