"Three months ago, Hezbollah's security apparatus detained a man named Mohammad Shawraba, who is a collaborator in its ranks," said the source, who spoke to AFP on condition of anonymity.
Hezbollah's investigation of Shawraba, who comes from the southern Lebanese town of Mahruna, revealed he had been working with Israel's Mossad intelligence service since 2007, the source said.
Hezbollah, the powerful Shiite movement whose opponents accuse it of being a "state within a state" in Lebanon, has refused to comment on the arrest.
According to the source, Shawraba was in charge of Hezbollah "unit 901", which coordinates the group's foreign operations.
Shawraba lived in Hezbollah's south Beirut stronghold but travelled frequently, said the source, adding that Israel's spy agency recruited him during a trip abroad.
"This member sabotaged some five security operations that Hezbollah had been planning abroad against Israeli-linked interests," said the source.
Hezbollah had been organising attacks to avenge the killing of its top military commander Imad Mughniyeh, who was assassinated in a 2008 car bomb attack in Damascus which the Shiite group blamed on Israel.
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Party chief Hassan Nasrallah had frequently threatened to carry out revenge attacks.
The source said Hezbollah launched a probe that eventually led to Shawraba's arrest following its attack on Israeli tourists in Bulgaria in 2012.
The 2012 bombing killed five tourists and the bus driver, but the militant group's suspicions were aroused when the bomber also died and his two accomplices were quickly identified.
Residents of Shawraba's village said they were shocked over the allegation.
"Shawraba is nearly 50 years old, and hasn't been here for a long time, but his family is known for its loyalty to Hezbollah," a Mahruna resident told AFP.
"For now, it's just a rumour, but if the movement officially announces his arrest, his family will disown him," he added.
Hezbollah has rarely been infiltrated by foreign intelligence agencies.
In 2012, the group arrested an official in its missiles unit named Abu Trab, who was accused of providing Israel with information about the locations of Hezbollah's rockets in southern Lebanon during a month-long 2006 war.
Iran-backed Hezbollah has a massive weapons arsenal. The group is represented in Lebanon's parliament and government.