The visitor said he spoke to Benedict several weeks ago, Zenit reported Wednesday.
"God has told me," Benedict was quoted as saying.
The visitor said Benedict described a "mystical experience" in which he received the message that it was time to give up the papacy and spend the rest of his life in prayer.
Benedict, 86, has lived quietly since leaving in March -- first at the papal summer residence, Castel Gandolfo, and then at the Mater Ecclesiae monastery in the Vatican.
He was the first pope to resign in hundreds of years and the first since Celestine V -- who stepped down in 1294, a few months after his election -- to do so voluntarily.
Benedict left the Vatican Sunday for the first time in months, Zenit -- a non-profit, pro-Catholic news agency -- said.
The news agency, citing sources in the Vatican, said Benedict, accompanied by the four women who look after his living quarters, traveled to Castel Gandolfo where he walked in the gardens and listened to a concert of classical piano music.