It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him); he said: When the Messenger of Allah (peace and blessings be upon him) came to stay with me, I saw that he always prayed four rak'ahs before Zuhr, and he said: "When the sun passes the meridian, the gates of the heaven are opened, and none of them is closed until Zuhr is prayed, so I love that a righteous deed of mine should be raised up in that hour."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4020