Abuja - The wife of ailing Nigerian President Muhammadu Buhari has left the country to visit him in London after first attending an African Union event, her office said. Aisha Buhari's office said late on Sunday that she was first heading to the AU summit in Addis Ababa for a "symbolic appearance" at a leaders' wives meeting.She will then head to Britain on Tuesday, a statement said, adding: ""She will convey to the president the best wishes of Nigerians and their fervent prayers for his quick recovery."Aisha Buhari first visited her 74-year-old husband in London on May 30 and returned to Nigeria on June 6, saying he was recovering fast. The head of state of Africa's most populous nation has spent most of this year in London receiving treatment for an unspecified medical condition.In January and February, he spent almost two months in Britain and on his return in early March said he had never been as ill.He left for the British capital again on May 7 for a second round of medical treatment. No date was given for his return. The presidency has repeatedly rejected rumours that Buhari is terminally ill or even dead but has refused to disclose his illness or what treatment he is having. Last week, opposition Ekiti state governor Ayodele Fayose advised Buhari to resign but his supporters rejected the suggestion. Vice-President Yemi Osinbajo has been acting on his behalf in accordance with the constitution. Buhari's absence has brought forward behind-the-scenes jostling for position for the 2019 presidential election at which the ailing leader is unlikely to stand. In 2010, Nigeria was plunged into months of political turmoil after president Umaru Musa Yar'Adua died in office following months of treatment abroad.