It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) concerning the four rak'ahs before Zuhr: (He narrates that) I said: O Messenger of Allah, what is this prayer that you always pray when the sun passes the meridian? He (peace and blessings be upon him) said: "O Abu Ayyub, the gates of the heaven are opened when the sun passes the meridian, and the gates of the heavens are not closed until Zuhr is prayed." I asked: Is there recitation in them? He (peace and blessings be upon him) said: "Yes." I asked: Are they separated by a salam? He (peace and blessings be upon him) said: "No."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4017