Traced to Ka'b al-Ahbar (automatic guess): a report of earlier scriptures, not a hadith of the Prophet ﷺ.
يأتيهم الخبر بعد فتحها، يعني القسطنطينية، فيرفضون ما في أيديهم، فيخرجون فيجدونه باطلا، لا يخرج الدجال إلا بعدها، تتعلق به حية إلى جانب البحر، ثم يخرج
The news comes to them after its conquest, meaning Constantinople, and they reject what they have in their hands, so they go out and find it to be false. The Dajjal will not come out until after that, and a snake clings to him by the side of the sea, then he comes out.
أبو أيوبأرطاة بن المنذرشريح بن عبيدكعبautomatic reading, unverified