Musnad Ahmad 30:3229

Bahz and 'Affan narrated to us, they said: Hammam narrated to us, he said: Qatadah was asked about a man who prayed one rak'ah of the morning prayer, then the sun rose—'Affan said: "Then the horn of the sun rose"—so he said: Khallas narrated to me, from Abu Rafi', that Abu Hurairah narrated to him, that the Messenger of Allah (ﷺ) said: "He shall complete his prayer."