In Ihya Uloom Id-Din of Imaam Ghazzali, may Allaah have mercy on him, he says that there are some optional prayers which can be performed on everyday of the week and nights. Is that really authentic or weak because he quotes Hadeeth related to the Prophet, sallallaahu alayhi wa sallam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Your question is not clear, however, we say that Allaah legislated some supererogatory prayers other than the obligatory prayers which a person performs. Indeed a man asked the Prophet about the prayers and he replied: "Five prayers during the day and the night." Upon hearing this the man said: "Am I obliged to perform any other prayer besides these?" The Prophet replied: "No, except if you want to perform supererogatory prayers."

So, these supererogatory prayers will be an endorsement for any deficiency in the obligatory prayers. Abu Hurayrah narrated that the Prophet said: "The first thing a person will be held accountable for on the Day of Judgment is his prayers. If they are complete, it will be written for him that they are complete, but if there is a deficiency in them, Allaah would say: "See if My Slave has any supererogatory prayers." Allaah would also say: "Make up the deficiency in My Slave's obligatory prayers from the supererogatory prayers;" and then other actions will be accounted for accordingly."  [An-Nasaa'ee and others]

For more benefit, please refer to Fatwa 84719 about some kinds of supererogatory prayers.

Allaah Knows best.