It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever catches one rak'ah of the Subh prayer before the sun rises, and then the sun rises, let him pray another (rak'ah) along with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1777