The Messenger of Allah ﷺ said, "Whoever prays one rakah of the dawn prayer before the sun rises, and then the sun rises, let him add another rakah to it."
حدثنا محمد بن جعفر وروح قالا حدثنا سعيد عن قتادة عن خلاس عن أبي رافع
أن رسول الله ﷺ قال من صلى من صلاة الصبح ركعة قبل أن تطلع الشمس ثم طلعت فليصل إليها أخرى
Its isnad is sahih. Khilas is Ibn Amr Hijri, who is trustworthy. The hadith is found in Daraqutni 1/382, and in Hakim 1/274 with two isnads, where he said, "Both are sahih," and Dhahabi agreed with him, and in Bayhaqi 1/379. All of them have similar wordings.