Imam Al-Nawawi's Riyad-us-Saliheen
Chapter 200
Sunnah of the 'Asr Prayer

1119. `Ali bin Abu Talib (May Allah be pleased with him)reported: The Prophet (sallallaahu ’alayhi wa sallam) used to perform four Rak`ah before the `Asr prayer, separating them with Taslim (i.e., offering blessings) on the favourite angels who are near Allah's proximity and the Muslims and the believers who come after them.
[At-Tirmidhi].

Commentary: The phrase "separating them with Taslim'' means that he would perform four Rak`ah in two couplets.

1120. Ibn `Umar (May Allah be pleased with them) reported: The Prophet (sallallaahu ’alayhi wa sallam) said, "May Allah have mercy on a man who performs four Rak`ah before the `Asr prayer.''
[Abu Dawud and At-Tirmidhi].

Commentary: These four Rak`ah can be performed in two couplets also, as was the practice of the Prophet (sallallaahu ’alayhi wa sallam), according to the preceding Hadith. It can be performed with one Taslim also. Both forms are correct and permissible. Some scholars are of the opinion that the former method is better. `Ulama' have stated that these four Sunnah of `Asr prayer are Ghair Mu'akkadah (optional prayers). Its importance is however evident from the fact that the Prophet (sallallaahu ’alayhi wa sallam) prayed for mercy on those who performed these Sunnah.

1121. `Ali bin Abu Talib (May Allah be pleased with him) reported: The Prophet (sallallaahu ’alayhi wa sallam) used to perform two Rak`ah before the `Asr prayer.
[Abu Dawud].

Commentary: We learn from this Hadith that one can also perform two Sunnah before `Asr prayer. But Sheikh Al-Albani has stated that the word "two Rak`ah" occurring in this Hadith is rare. Four Rak`ah are secure and should, therefore, be preferred.