Assad also vowed that his country would always back resistance to Israel until the liberation of Palestine.
Assad's remark came during an hour-long meeting with a Hamas delegation, the kick-off of a regional tour meant to thank countries that stood beside the Islamic movement.
"Assad congratulated the Palestinian people and the heroic resistance, on top of which Hamas, for the legendary victory and steadfastness that drove out the Zionest enemy from the Strip," a Damascus-based Hamas source told CBS News.
"The Syrian leader underlined that it was the duty of his country to support the resistance and that it would keep doing so until achieving the Palestinian national goals: expelling the occupation and establishing an independent Palestinian state with Jerusalem as its capital," he quoted Assad as telling Mishaal.
The Hamas leader voiced his gratitude to Syria's statements, and considered Damascus a "partner in the victory."
A Syrian official source quoted Assad as telling the delegation that the "defeat of Israel was due to the adherence to the Palestinian rights." He refused to give more details.
The Palestinian faction received a huge political boost a week ago when its leaders attended the top-level gathering of the so-called "hardline" Arab countries (as well as Iran's Mahmoud Ahmadinejad, who is viscerally hostile to Israel) for the first time.
Mishaal, along with the leaders of Islamic Jihad and the Popular Front for the Liberation of Palestine (which were also fighting Israel in Gaza), flew to Doha from Damascus on the Emir of Qatar's private plane.
Israel accuses Syria and Iran of supporting and arming Hamas, an accusation they deny.
Israel and Hamas have observed their own cease-fires since last Sunday when Israel ended Operation Cast Lead, leaving a trail of devastation and 1,330 Palestinians dead, half of them children and women.
Egypt is trying to secure a durable cease-fire and the reopening of border crossings, followed by Palestinian reconciliation talks.
Hamas threatened to resume fighting if Israel does not reopen the crossings, and Israel warned it would strike again if Hamas is allowed to rearm.
U.S. President Barack Obama has called on Israel and Hamas to take steps aimed at ensuring that the cease-fire in place in Gaza will endure, and said he would aggressively seek a lasting peace between Israel and the Palestinians, while also always defending Israel's "right to defend itself."
While European nations and, of course, the United States, refuse to talk to Hamas until it recognizes Israel, renounces violence and accepts interim peace deals signed by the Palestine Liberation Organization, many observers say the group should be engaged in any political process, especially after the Gaza war.
By CBS News' George Baghdadi in Damascus.