Musannaf Ibn Abi Shayba 3:3564

Abu al-Ahwas narrated to us, from Sa'id ibn Masruq, from al-Musayyab ibn Rafi', who said: Abu Ayyub al-Ansari said: "O Messenger of Allah, what are these four rak'ahs that you persist in performing before the noon (Dhuhr) prayer?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, the gates of heaven are opened when the sun passes its zenith, and they do not close until the prayer is established. So, I love that a good deed of mine should ascend during that time."