Short answer
In the Indian subcontinent and among Hanafis, salat al-awwabin usually means six voluntary rakats after Maghrib, and it is recommended. In an authentic hadith, though, the Prophet ﷺ gave the name "prayer of the awwabin" to the duha prayer in the late morning. Both are good nafl prayers; the hadith promising great reward for six rakats after Maghrib is weak.
Six rakats after the Maghrib fard and sunnah, in sets of two, are mustahabb, and Hanafi texts call them salat al-awwabin. Some scholars count the two sunnah of Maghrib within the six. Nothing about them is wajib, and praying fewer is fine.
The four schools agree on this.
Explanation
Awwabin means "those who often turn back to Allah". The name is used for two different voluntary prayers:
- Duha: the Prophet ﷺ said that the prayer of the awwabin is when the young camels feel the heat of the sand, that is, the late morning. This is in Sahih Muslim, and it refers to the duha prayer.
- After Maghrib: Hanafi books, and common practice in South Asia, use the name for six rakats after Maghrib.
Is the Maghrib prayer authentic? Praying between Maghrib and Isha is well established: Anas explained the verse about those who "used to sleep but little of the night" as people who prayed between Maghrib and Isha. The specific report that six rakats after Maghrib equal twelve years of worship is graded very weak by hadith scholars, so do not rely on that number. Hanafi scholars still recommend the six, because voluntary prayer in this time has sound support and weak reports may be used to encourage virtues.
How to pray it: after the two sunnah of Maghrib, pray four or six more rakats in sets of two, with any surahs you know and the intention of nafl. There is no special du'a or surah that must be recited. If you are busy, two rakats are better than none.
All four schools encourage voluntary prayer between Maghrib and Isha; they differ only in the name and number. See all the prayers in Islam and how to pray a nafl prayer.
The evidence
Quran, Surah Adh-Dhaariyat 51:17
كَانُوا۟ قَلِيلًۭا مِّنَ ٱلَّيْلِ مَا يَهْجَعُونَ ﴿١٧﴾
They used to sleep but little of the night,
Anas linked this verse to prayer between Maghrib and Isha.
Sahih Muslim 748
Sahih... صلاة الأوابين حين ترمض الفصال ...
"The prayer of the awwabin is when the young camels feel the heat."
The authentic use of the name, for the late-morning duha prayer.
Sunan Abi Dawud 1322
Grading to be checked... كانوا يصلون فيما بين المغرب والعشاء ...
"Anas said about the verse: they used to pray between Maghrib and Isha."
Voluntary prayer between Maghrib and Isha has support from the early Muslims.
Where the ruling is recorded
- Nur al-Idah with Maraqi al-Falah, Chapter on nawafil. Six rakats after Maghrib are recommended among the voluntary prayers.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on witr and nawafil. The six after Maghrib called salat al-awwabin, and the use of weak reports for virtues.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.