Musnad Abu Ya'la al-Mawsili 54:5

Al-Hakam ibn Musa Abu Salih narrated to us, Baqiyyah narrated to us, he said: Abu al-Hajjaj al-Mahri narrated to me, Zaban ibn Fa'id narrated to us, from Sahl ibn Mu'adh, from his father, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs the dawn prayer (Fajr), then sits remembering Allah until the sun rises, Paradise becomes mandatory for him."