Boxing legend Muhammad passed away as Ali was in "grave condition" at Phoenix-area hospital with a respiratory problem. A source close to the family said, as speculation swirled about his health.
Asked about Ali's condition, the source said: "It's extraordinarily grave. It's a matter of hours."
The 74-year-old former heavyweight champion's family has gathered by his bedside just a day after he had been hospitalized in what spokesman Bob Gunnell said Thursday was to be a "brief" stay.
Ali has been suffering with advanced Parkinson's disease in recent years — an illness that experts say can lead to complications and affect the breathing of patients.
The boxing legend was previously hospitalized for several days in December 2014 and January 2015 after suffering a severe urinary tract infection initially thought to be pneumonia.
Ali's last public appearance was in April at the "Celebrity Fight Night" gala in Arizona, a charity that benefits the Muhammad Ali Parkinson Center.