Sunan An-Nasai, Chapter: The Book of the Times (of Prayer)

"I asked Lahiq about the two Rakahs before sunset. He said: "Abdullah bin Az-Zubair used to pray them, and Muawiyah sent word to him asking: 'What are these two Rakahs at sunset?' He had to refer to Umm Salamah, and Umm Salamah said: 'The Messenger of Allah (saw) used to pray two Rakahs before Asr, then he was distracted and did not pray them, so he prayed them when the sun set, and I never saw him pray them before or after that.'" (Sahih)