Praise be to Allaah.
The regular Sunnah prayers fall into two categories:
Sunnahs which are done before the fard prayer – two rak’ahs before Fajr and four before Zuhr.
Sunnahs which are done after the fard prayer – two rak’ahs after Zuhr, two rak’ahs after Maghrib and two rak’ahs after ‘Isha’.
Concerning their virtue, it was narrated that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever prays twelve rak’ahs during the night and day, a house will be built for him in Paradise: four before Zuhr and two after, two rak’ahs after Maghrib, two rak’ahs after ‘Isha’ and two rak’ahs before Fajr prayer.”
Narrated by al-Tirmidhi, no. 380; classed as saheeh by al-Albaani in Saheeh al-Jaami’, 6362.
The time for a Sunnah prayer that comes before a fard prayer is from the beginning of the time for that prayer, until the iqaamah.
The time for a Sunnah prayer that comes after a fard prayer is from after the salaam at the end of the fard prayer until the end of the time for that prayer.
Ibn Qudaamah (may Allaah have mercy on him) said: “The time for every Sunnah prayer that comes before a fard prayer is from the beginning of the time for that prayer, until the prayer is done, and the time for every Sunnah prayer that comes after a fard prayer is from after the prayer is done until the end of the time for that prayer.”
Based on this, it is not correct to pray the Sunnah prayer before the time for the fard prayer begins, rather you should wait until the time for that prayer comes, then pray it.
Praise be to Allah;
“Allaah will build a house in Heaven for whoever is diligent in observing 12 Sunnah Rak’aat (as follows): 4 Rak’aat before and 2 after the Dhuhr (Midday) Prayer, 2 after the Maghrib (Sunset Prayer), 2 after the ‘Ishaa’ (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-Jaami’.
‘Anbasah ibn Abi Sufyan quoted Umm Habibah as saying “Allaah’s Messenger (May peace and blessings be upon him) said:
“A house will be built in Heaven for one who prays 12 Rak’aat in a day and evening as follows: 4 Rak’aat before and 2 after the Dhuhr Prayer, 2 after the Maghrib Prayer, 2 after the ‘Ishaa’ Prayer and 2 before the Fajr Prayer.” Narrated by at-Tirmizi 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-Jaami’.
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’aat before the ‘Asr Prayer. The 4 Rak’ahs are of less reward and significance in importance of adhering to them compared to the “sunnan al-rawaatib” described above. The 4 Rak’ahs are the ones intended by the Prophet (May peace and blessings be upon him) in saying: “May Allaah have mercy on one who prays 4 raka’aat before the ‘Asr Prayer.” Narrated by at-Tirmidhi no. 395 and he declared it a hassan and ghareeb 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.
Allaah, the Exalted, knows best.