Surprisingly enough, the source added, the man had died in custody last month.
He said the deceased Muslim Brotherhood member was one of 30 other members against whom the jail sentences were upheld on Sunday.
They were all charged with rioting and participating in violence, the source said.
This is the fifth time an Egyptian court issues prison sentences against dead supporters of ousted president Mohamed Morsi in a year and a half since his ouster in July last year.
In August of this year, an Egyptian court sentenced a Muslim Brotherhood member to death on charges of killing a policeman and attacking a police station, even as the member had died six months earlier.
Sunday's verdict against the deceased Morsi supporter can be appealed, a judicial source said.
Called Tarek Ghandour, the deceased suspect was a physician by profession. He died in mid-November after suffering health complications inside his prison cell in the Nile Delta.
The Muslim Brotherhood held Egyptian prison authorities responsible for his death, attributing it to what it called "negligence."
The movement said officials at al-Koum Prison in the Nile Delta had refused to transfer Ghandour to a hospital, even as he suffered internal bleeding.
The authorities, for their part, denied the accusation, saying the man's condition was "beyond treatment."
Morsi, this country's first freely elected president, was ousted by the army following protests against his government in July of last year.
Thousands of his supporters were thrown in jail ever since on charges of participating in and inciting violence.