Musnad Ahmad 30:3209

Muhammad ibn Ja'far and Rawh narrated to us, they said: Sa'id narrated to us, from Qatadah, from Khallas, from Abu Rafi', from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs one rak'ah of the morning prayer before the sun rises, then it rises, let him perform another one to it."