His health has been a major cause of concern in a country where there are fears that a power vacuum could affect its recovery from recession.
In a brief message, the president said "there is no cause for worry".
Mr Buhari, 74, has left Vice-President Yemi Osinbajo in charge, just as he did when he left for London in January for seven weeks of medical leave.
Mr Osinbajo was widely praised at the time for his performance as acting president.
