Ali, who turns 73 on Saturday, is in a stable condition at an undisclosed hospital, The Courier Journal and WDRB radio in his hometown of Louisville reported.
The visit was for follow-up care, the reports quoted his spokesman as saying.
Ali was released from hospital on January 7 where he had been treated for a urinary tract infection since last month.
He has suffered from Parkinson's disease for the last 30 years.