Musnad Ahmad 1060:68

Abdullah ibn al-Walid narrated to us, Sufyan narrated to us, al-A’mash narrated to us, from al-Musayyab ibn Rafi’, from a man, from Abu Ayyub, who said: The Prophet, peace and blessings of Allah be upon him, used to pray four [rak'ahs] before the Dhuhr prayer. It was said to him: "You pray a prayer that you persist in." He said: "Indeed, the gates of the heaven open when the sun passes its zenith, and they do not close until the Dhuhr prayer is performed. So, I love that a good deed of mine should ascend to the heaven [during that time]."