"I prayed for my grandson Barack to convert to Islam," said Sarah Omar, 88, in an interview on Thursday with Al-Watan Saudi daily held in Jeddah after she had performed hajj.
The paper said that Omar was in Saudi Arabia on pilgrimage along with her son, Obama's uncle Saeed Hussein Obama, and four of her grandchildren.
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Omar told the newspaper that she could only discuss hajj matters and would not comment on Obama's politics.
The family appeared to have been hosted by the Saudi government for hajj. Saeed thanked King Abdullah for his "kind hospitality," the paper said.
US opinion polls in August revealed that roughly one in five Americans believe that Obama is a Muslim, a claim categorically denied by the White House which has maintained that he is a "committed Christian."