Bill Gross, the bond market's most renowned investor, quit Pimco for distant rival Janus Capital Group Inc. on Friday, the day before he was expected to be fired from the huge investment firm he co-founded more than 40 years ago.

Gross, 70, had been clashing with the firm's executive committee and had threatened to resign multiple times, a source familiar with the situation said. The committee had planned to accept his latest resignation from the post of chief investment officer Saturday.

The surprise development, which rattled the U.S. bond market, came the day before Pimco and its parent, German insurer Allianz SE, planned to dismiss Gross, the source said.