Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports from the Prophet (peace and blessings be upon him): "When one of you prays one rak'ah of the Subh (prayer) and then the sun rises, let him pray another (rak'ah) along with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1778