Musnad Ahmad 907:40

Husayn ibn Ali narrated to us, from Za'idah, from Simak, from Jabir ibn Samurah, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) prayed the dawn prayer, he would sit in his place of prayer until the sun rose." He said: "And he used to recite in the dawn prayer 'Qaf. By the Glorious Qur'an,' and his prayers after that were light."