Someone else with more expertise, feel free to correct me, but the short answer is Constantinople. You need to conquer Constantinople if you wish to move people or maintain a conquest from Anatolia into Europe.

It's worth pointing out that Muslims did try to go through Constantinople during their initial phase of expansion, but were stopped at the gates of Constantinople in 674 under the first Umayyad caliph Mu`awiya. This marked the high water mark of Muslim expansion into Anatolia until the Turks, without Constantinople the Umayyads couldn't invade Thrace or maintain their conquest of the port regions of Anatolia that relied on Constantinople to resupply them in the event of sieges.

You might ask why didn't the Ummayyads simply conquer the Anatolian hinterland like the Turks did, the issue was that they simply didn't have the manpower or will to hold down what was a mostly Christian population while fighting the Byzantines. One such example being the Mardaites who were a Christian sect living in the Syrian/Anatolian highlands that were raiding Mu'awiyas supply lines into Anatolia and after the failure of the siege of 674 a cause for Mu'awiya to withdraw.

In 718 another Umayyad Caliph would try to siege Constantinople, and meet a similar failure. He would be the last Caliph to try, and after his failure the Umayyads were overthrown a few decades later by the Abbasid caliphs. Whose power base was in Iraq, thus more focused on Central Asian and Indian conquests versus Anatolia. In fact during the Abbasids the Byzantines had a resurgence which saw them slowly creep all the way back to Syria until the Turks arrived.

In comparison, the Arabs were invited to land in Spain by a Visigothic noblemen who aided the initial conquest of Tariq Bin Ziyad. The difference lies in that the Visigoths didn't have a strong central government centered on a defensible geographic location to serve as a buffer. The Visigothic nobleman who helped Tariq land, negated the geographic defense that was Gibraltar. So the Arabs were able to easily sweep through Spain. But, they did encounter major geographic boundaries in Northern Spain that did prevented them from wiping out the remnants of the Visigothic kingdoms and were obviously unable to continue a war with France due to the geographic barrier that is the Pyrenees.

So in short, Constantinople created a defensible position that was used to maintain control over Anatolia and prevent Muslim conquests from continuing into Thrace. Whereas in Spain, the Visigothic civil war gave an opening for the Arabs to sweep through most of Spain unhindered until geographic limitations stopped their conquest.

Answered on my phone, so apologies for any typos.

Edit: changed Vandals to Visigoth.