You can pray voluntary to units of prayer after Asr. As a matter of fact its is sunnah, but many people do not know.
Al-Albani (may Allah rest him in Firdous) said,” This is a prayer which is forgotten and should be revived.”[ Saheehah hadeeth hadeeth 3174]
عَائِشَةَ قَالَتْ صَلاَتَانِ مَا تَرَكَهُمَا رَسُولُ اللَّهِ -صلى الله عليه وسلم- فِى بَيْتِى سِرًّا وَلاَ عَلاَنِيَةً رَكْعَتَانِ قَبْلَ الْفَجْرِ وَرَكْعَتَانِ بَعْدَ الْعَصْرِ.
1.Aisha (may Allah be pleased with her) said: “There are two prayers that the messenger of Allah (peace and blessings be upon him) never neglected to pray in my house secretly or publicly: Two Rak’ah before Fajr and two Rak’ah after Asr.” [Bukhari 592]
ائِشَةُ رضى الله تعالى عنها مَا دَخَلَ عَلَىَّ رَسُولُ اللَّهِ -صلى الله عليه وسلم- بَعْدَ الْعَصْرِ إِلاَّ صَلاَّهُمَا.
2. Aisha (may Allah be pleased with her) said: Anytime Allah’s messenger (peace and blessings be upon him) came to my house after Asr he prayed two Rak’ah [Bukhari 593 ]
The scholars’ analysis about these two Rak’ah:
1. Imam At-Tahawee (may Allah have mercy on him) said, “Some people see no harm in praying two Rak’ah after Asr and consider them a Sunnah.” [Sharh Ma’anee Al-Athar 1-301]
2. Imam Ibn Hazm (may Allah have mercy on him) said,” …Third: If this hadeeth is authentic it’s a proof for us. This hadeeth tells us that the Prophet (peace and blessings be upon him) prayed two rak’ah after Asr. On the contrary if it wasn’t allowed or disliked, Allah’s messenger (peace and blessings be upon him) wouldn’t have prayed these two rak’ah. His actions are guidance and correct, regardless whether he does it once or a thousand times.” [ Al-Muhalla]
3. Ibn Hajr (may Allah have mercy on him) said, “ To cling to these narrations establish the permissibility to offer a supererogatory prayer after Asr in any respect as long as it isn’t intended to be done during sunset.” [ Fath 2/85 ]
4. Shaykh Nasirud Deen Al-Albani (may Allah have mercy on him) said,” Among the accepted errors in the books of Fiqh is the prohibition of these two rak’ah and not mentioning them among the supererogatory exercises of devotion. Nevertheless, the Prophet (peace and blessings be upon him) preserved these two Rak’ah the same way he preserved the two before Fajr. There isn’t any evidence for the two Rak’ah after Asr’s abrogation or their being special for the Prophet (peace and blessings be upon him) only! How could it be, when Aisha (may Allah be pleased with her), the most knowledgeable person about these two rakah’ safeguarded them as well as other companions and pious Salaf.[ Silsilatus Saheehah 7/528]
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