Musnad Ahmad 1060:54

Yahya ibn Adam narrated to us, Sharik narrated to us, from al-A’mash, from al-Musayyib ibn Rafi’, from ‘Ali ibn al-Salt, from Abu Ayyub al-Ansari: That he used to pray four rak’ahs before the Dhuhr prayer. It was said to him, "You persist in this prayer." He said, "I saw the Messenger of Allah, peace and blessings of Allah be upon him, doing it, so I asked him about it. He said, 'It is an hour in which the gates of the heaven are opened, so I love that a righteous deed of mine should ascend during it.'"