Sunnah Prayers
Question

What are the confirmed sunnah prayers (Al-Sunan Al-Rawatib)? And when should they be performed?
Praise be to Allah.The answer to your question about al-sunan al-rawatib is a Hadith by the Prophet (may peace and blessings be upon him) in which he said:
"Allah will build a house in Heaven for whoever is diligent in observing 12 Sunnah Rak'at (as follows): 4 Rak'at before and 2 after the Zhuhr (Midday) Prayer, 2 after the Maghrib (Sunset Prayer), 2 after the ‘Isha (Evening) Prayer and 2 before the Fajr (Dawn) Prayer." (Hadith sahih narrated by at-Tirmidhi No. 379 and by others. Hadith No. 6183 in Sahih al-Jami’.)
Anbasah ibn Abi Sufyan quoted Umm Habibah as saying "Allah’s Messenger (may peace and blessings be upon him) said:
"A house will be built in Heaven for one who prays 12 Rak'at in a day and evening as follows: 4 Rak'at before and 2 after the Zhuhr Prayer, 2 after the Maghrib Prayer, 2 after the ‘Isha Prayer and 2 before the Fajr Prayer." (Narrated by at-Tirmidhi under No. 380. He said: The Hadith narrated by Anbasah quoting Umm Habibah in this chapter is a hassan and sahih hadith. It is under No. 6362 in Sahih al-Jami’.)
The ‘Asr (Late-Afternoon) Prayer has no routine Sunnah (basic recommended voluntary prayer). However, it is mustahab (preferable and recommended) that one prays 4 Rak'at before the ‘Asr Prayer. The 4 Rak'ahs are of less reward and significance in importance of adhering to them compared to the "sunan al-rawatib" described above. The 4 Rak'ahs are the ones intended by the Prophet (may peace and blessings be upon him) in saying: "May Allah have mercy on one who prays 4 rak’at before the ‘Asr Prayer." (Narrated by at-Tirmidhi no. 395 and he declared it a hassan and gharib Hadith. Al-Albany rated the Hadith as hassan in Sahih al-Jami’ No. 3493.)
All the foregoing 4-Rak'ah voluntary prayers are to be prayed two at a time according to Imam ash-Shafi'i and Imam Ahmad.
For more about sunnah prayers, please see these answers: 65746 , 181043 , 150006 , 187769 , and 297  
Allah, the Exalted, knows best.