The president's letter is a notification of his return to office after transmitting his executive powers before he travelled.
According to a press release signed by the president's spokesperson, Femi Adesina, Buhari forwarded the notification letter to the National Assembly on Monday.
According to Adesina, the letter read in part, "In compliance with Section 145 of the 1999 Constitution (as amended), I write to intimate you that I have resumed my functions as President of the Federal Republic of Nigeria with effect from Monday, 21st August, 2017, after my medical follow-up in the United Kingdom."
ALSO READ: President's full speech to Nigerians [VIDEO]
While speaking on Sunrise Daily earlier on Monday, Adesina had said the letter does not need to be read by lawmakers before it comes into effect.
According to him, despite that the nation's lawmakers are currently in recess, "Transmitting it is what the constitution requires. Once it is sent, he becomes president automatically. They don't have to read it or debate it, No. What the Constitution requires is for him to send it. Once it is sent, he assumes office."
When the president left the country on May 7 to resume treatment for an undisclosed illness in London, he transmitted his executive powers to Vice President Yemi Osinbajo.
After staying in London for 103 days, the president returned to Nigeria on Saturday to widespread jubilation in Abuja and relief around the country.