Buhari made this known through his twitter handle. The president tweeted:

It is with profound grief and sadness that we received the news of the death of President Pierre Nkurunziza. President Nkurunziza was a true patriot that steered the country through turbulent times with wisdom and foresight.

At this time of great pain & loss,the Government and people of Nigeria as well as myself, express our deepest condolences to the Government and people of Burundi. Our thoughts & prayers also go out to the family of the President. May God grant them the fortitude to bear the loss.