Madonna has thanked Sir Bob Geldof for inspiring her to adopt a Malawi baby. The 'Hung Up' singer apparently decided to adopt 14-month-old David - who she has temporary custody of for now - when she visited the African country after being encouraged to do so by Sir Bob. Madonna, 48, is said to have sent a heart-felt letter to the former Boomtown Rats singer - who has been involved with aid work in Africa for years and organised the global Live 8 charity concerts in 2005 - and made a sizable donation to his Band Aid charity. A source told Britain's The Sun newspaper: "She feels that she is forever in Sir Bob's debt for having introduced her to the country where she eventually met her beloved son. She is extremely grateful to him for bringing David into her life and feels she owes him a lot. She sent Bob a lovely thank you letter to extend her gratitude."Sir Bob and Madonna reportedly fell out briefly before the Live 8 concerts last year when Bob announced she would be performing at the London show before she had agreed to do so. However, the pair quickly sorted out their differences and Bob suggested she visit Africa, with her husband Guy Ritchie, as the pair chatted backstage after her performance.