In the Koran there are stories told about Jesus. But is it known which source is used for those stories. In the christian bible the story of Jesus came from old papyrus rolls from around the years 200 AD. But how did they came in the Koran?
Allah (SWT) revealed those stories to his Messenger (PBUH) through Angel Gabriel (Jibreel).
Allah (SWT) says in Surat Hud:
وَكُلًّا نَّقُصُّ عَلَيْكَ مِنْ أَنبَاءِ الرُّسُلِ مَا نُثَبِّتُ بِهِ فُؤَادَكَ ۚ وَجَاءَكَ فِي هَـٰذِهِ الْحَقُّ وَمَوْعِظَةٌ وَذِكْرَىٰ لِلْمُؤْمِنِينَ ﴿١٢٠
And all that We relate to you (O Muhammad SAW) of the news of the Messengers is in order that We may make strong and firm your heart thereby. And in this (chapter of the Qur'an) has come to you the truth, as well as an admonition and a reminder for the believers. (120)