Musnad Abu Ya'la al-Mawsili 54:13

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