Musnad Ahmad 1060:35

Abu Mu'awiyah narrated to us, Ubaydah narrated to us, from Ibrahim, from Sahm ibn Minjab, from Qaza'ah, from al-Qartha', from Abu Ayyub al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to persist in performing four rak'ahs when the sun passed its zenith. He said: So I said, "O Messenger of Allah, what are these rak'ahs that I see you persisting in?" He said: "Indeed, the gates of the heaven are opened when the sun passes its zenith, and they do not close until the Dhuhr prayer is performed, so I love that some good of mine should ascend during them." He said: I said, "O Messenger of Allah, do you recite in all of them?" He said: "Yes." He said: I said, "Is there a separating salutation in them?" He said: "No."