It is narrated from Sayyiduna Ma'qil bin Yasar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Surah al-Baqarah is the peak and summit of the Qur'an; with every one of its verses eighty angels descended. And 'Allah - there is no true god except Him, the Ever-Living, the Sustainer of all' [Al-Baqarah: 255] was brought out from beneath the Throne and then joined to Surah al-Baqarah. And Surah Ya-Sin is the heart of the Qur'an; no man recites it seeking Allah and the abode of the Hereafter except that Allah Almighty forgives him. And recite it over your dying."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17645