"The leaders stressed the importance of stability... the commitment (of the Lebanese) not to resort to violence and the need to place the country's interests above all sectarian interests," said a communique issued after a mini-summit between Assad, the Saudi monarch and Lebanese President Michel Sleiman.
The statement also stressed the need to "resort to legal institutions and Lebanon's unity government to resolve any differences."
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The pair made the hours-long visit in a united bid to ease tensions over reports of an impending indictment against members the Shiite Islamist party Hezbollah for the 2005 murder of Lebanese ex-premier Rafiq Hariri.
The United Nations hailed their visit as potentially "historic" for Lebanon.
"The visits of these Arab heads of state will be enormously important and beneficial for Lebanon's stability and future," Michael Williams, the UN's special coordinator to the country, said in a statement.
Fears of renewed conflict in Lebanon rose last week after Hezbollah leader Hassan Nasrallah revealed he knew the UN tribunal probing Hariri's killing was poised to indict members of his party, which is backed by Syria and Iran.
Nasrallah made it clear he would not accept such a scenario, accusing the tribunal of being politicised and part of an Israeli plot.
Hezbollah deputy Hassan Fadlallah said later he believed the issue of Special Tribunal for Lebanon based in The Hague was the main focus of Friday's talks between the three leaders.
"The latest developments concerning the international tribunal were at the heart of the summit," Fadlallah told AFP.
Analysts say that in addition to threatening civil peace, an indictment of Hezbollah members would deal a blow to the party's reputation and destabilise Hariri's unity government.
The joint communique by Assad and King Abdullah urged Lebanese parties to "pursue the path of appeasement and dialogue and to boost national unity in the face of outside threats," referring to Israel.
Asked about the outcome of the brief talks as he left the presidential palace, the Syrian leader gave a thumbs up and said: "The discussions were excellent."
Assad was making his first visit to Lebanon since Hariri's assassination soured bilateral ties and forced the pullout of Syrian troops from Lebanon after a 29-year presence.
Damascus has consistently denied accusations it had a hand in the killing.
Relations between the two countries have been on the mend since 2008, when diplomatic ties were established for the first time. Prime Minister Saad Hariri, son of the slain Sunni leader, has also made four trips to Syria in the past eight months.
Saudi Arabia, a staunch supporter of the Hariri family, has played a key role in the rapprochement between the Arab neighbours.
Saudi and Syrian flags were on display throughout the Lebanese capital on Friday along with huge portraits of the Saudi monarch.
Security was also tight, with additional army and police deployed.
In addition to attending the summit at the presidential palace, the Saudi monarch paid a brief visit to Hariri's home in downtown Beirut where he met with religious leaders and other officials.
Syrian Foreign Minister Walid Muallem, who accompanied Assad to Beirut, met separately with several Hezbollah deputies.
Assad last visited Lebanon in 2002, and King Abdullah is the first Saudi monarch to visit the country since 1957.
Also arriving in Beirut on Friday was the ruler of Qatar, Sheikh Hamad bin Khalifa al-Thani, for a two-day visit that will take him to parts of the war-torn south whose reconstruction the emirate is helping to finance.
