Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) used to pray four rak'ahs before Zuhr. He was asked (what this was), and he said: I saw the Messenger of Allah (peace and blessings be upon him) praying them, so I asked him, and he (peace and blessings be upon him) said: "It is an hour in which the gates of the heaven are opened, so I love that a righteous deed of mine should be raised up in it."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4022