The source, who spoke on condition of anonymity, said between four and five members came into contact with chemical agents while searching a group of rebel tunnels in the Damascus suburb of Jobar over the weekend.
On Saturday, Syrian state television said Syrian soldiers found chemical agents in Jobar and that some had suffocated while entering the tunnels.
The Hezbollah fighters were transported to Lebanon for treatment and are currently in a Beirut hospital in stable condition, the source said.
The source did not say what the chemical agent the fighters encountered was except that it was dispersed in the form of a gas. U.N. inspectors are investigating claims of a chemical weapons attack in eastern Ghouta, outside the Syrian capital .
In May, Hezbollah’s Secretary-General Sayyed Hasan Nasrallah said fighters from his party were fighting alongside forces loyal to President Bashar Assad in Syria . Nasrallah said the move aimed at protecting Lebanon from a jihadist threat.