Hammam says that Qatadah was asked about a man who prayed one rak'ah, then the edge of the sun rose. He said: Khilas narrated to me from Abu Rafi' that Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrated that the Prophet (peace and blessings be upon him) said: "He should complete his prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1776