Hammam said, "I found in my book a narration from Bashir bin Nuhayk, and I do not think it is anything other than from Nadr bin Anas, from Bashir bin Nuhayk, from Abu Hurairah, that the Messenger of Allah ﷺ said, 'Whoever prays one rakah of the dawn prayer and then the sun rises, let him complete his prayer.'"
حدثنا بهز حدثنا همام حدثنا قتادة عن بشير بن نهيك
همام وجدت في كتابي عن بشير بن نهيك ولا أظنه إلا عن النضر بن أنس عن بشير بن نهيك عن أبي هريرة أن رسول الله ﷺ قال من صلى ركعة من الصبح ثم طلعت الشمس فليتم صلاته
Its chain of transmission is sahih, despite the doubt expressed by Hammam in it. But this has no bearing, as will be explained. The hadith was narrated by Hakim (1:274) via Ahmad bin Atiq Marwazi, who said, Muhammad bin Sinan Awaqi told us, Hammam told us, Qatada told us, from Nadr bin Anas, from Bashir bin Nuhayk, from Abu Hurairah, from the Prophet ﷺ, who said, "Whoever prays one rakat of the dawn prayer and then the sun rises should complete the dawn prayer."Hakim said, This is a hadith that meets the standard of the two sheikhs, if it is preserved with this chain of transmission. For this Ahmad bin Atiq Marwazi is trustworthy, except that he narrated it another time with a different chain. He then narrated it via Ahmad bin Atiq, from Muhammad bin Sinan, from Hammam, from Qatada, from Khilas, from Abu Rafi, from Abu Hurairah, as a raised hadith, with the same wording as here.Then he said, Both chains are sahih only, they both relied on Khilas bin Amr as a corroborating witness. Dhahabi agreed with him on everything he said.The narration of Khilas bin Amr appeared previously at 7215, where we established its soundness and pointed to Hakim's statement and to this chain here as well. It appears that Hammam found the chain in his book missing the link through Nadr bin Anas, as he explicitly stated here, but he judged it more likely that it was in fact established in the chain. So he narrated it in this manner, then became certain of what he had judged more likely, and narrated it with certainty, dropping the doubt, as the narration of Hakim indicates.The meaning of the hadith is sahih and firmly established, having appeared many times before. See 7785 and the narrations we referred to there.