Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.008A.
Section : Reports concerning eight rak’at.
Umm Habibah reports that the Prophet said: “Whoever prays four rak’at before zuhr and four after it, Allah will forbid that his flesh be in the fire.” This is related by Ahmad, Abu Dawud, an-Nasa’i, Ibn Majah, and at-Tirmizhi who calls it sahih.
It is related that Abu Ayyub al-Ansari would pray four rak’at before zuhr. The people said to him: “You made that prayer continue too long!” He said: “I saw the Messenger of Allah doing so. I asked him about it and he said: ‘It is a time in which the doors of the heavens are opened and I wish that my good deeds be raised (to heaven) during it.'” This is related by Ahmad with a good chain.