He was speaking to dignitaries in the capital, Abuja, after returning from seven weeks of medical leave in the UK.
His absence led to some anxiety in Nigeria with people speculating about the president’s health.
Vice-President Yemi Osinbajo was left in charge and Mr Buhari’s spokesperson has tweeted that he will formally return to power on Monday.
The 74-year-old flew into an air force base in the northern city of Kaduna from London on Friday morning.
Despite talking about how he felt, Mr Buhari did not reveal any details of his condition.
He did say he was “feeling much better now” but added that further medical checks would be required.
At the end of his nine-minute speech, Mr Buhari said that he deliberately came back towards the weekend “so that the vice-president will continue and I will continue to rest”.
One of his advisers tweeted that he was joking and his office has clarified that the formal constitutional procedure for him to retake the reins of power will happen on Monday.
An official statement on Thursday said President Buhari had left Nigeria for a “vacation, during which he had medical check-ups”.
“The holiday was extended based on doctors’ recommendation for further tests and rest.”
The president handed power over to Mr Osinbajo in January before he left for the UK.
The country is currently suffering from its worst economic crisis in years, following a sharp fall in the price of oil, its major export.
It is the second time in less than a year that Mr Buhari, who won elections in 2015, has sought medical assistance overseas. Last June, he spent nearly two weeks, again in London, for treatment for an ear infection.
Little information has been made public about his latest stay in the UK capital, but he was pictured on Thursday meeting the most senior cleric of the world Anglican congregation, Archbishop of Canterbury Justin Welby.