Did Prophet Muhammad Pray 6 Rak`ahs after Maghrib?
Question

Is it true that the Prophet (peace and blessings of Allah be upon him) prayed six Rak`ahs in the Sunnah prayer of Maghrib?
Praise be to Allah.The regular Sunnah prayer of Maghrib is two Rak`ahs only. But there is a report concerning the virtue of praying six Rak`ahs after Maghrib. 
At-Tirmidhi (435) and Ibn Majah (1167) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Whoever prays six Rak`ahs after Maghrib and does not say anything bad in between them, will have a reward equal to the worship of twelve years.” 
At-Tirmidhi said: “This Hadith of Abu Hurayrah is a Hadith which we know only from the Hadith of Zayd ibn Al-Hubab from `Umar ibn Abi Khath`am. I heard Muhammad ibn Isma`il [Al-Bukhari] say:The Hadith `Umar ibn `Abdullah ibn Abu Khath`am is to be rejected, and he classed it as very inauthentic. 
Al-Albani (may Allah have mercy on him) said in his book Da`if At-Tirmidhi: It is very inauthentic. 
There are other Hadiths which encourage praying between Maghrib and `Isha’, but they are all inauthentic. (See: Da`if At-Targhib wat-Tarhib (332-335) 
But it is proven that the Prophet (peace and blessings of Allah be upon him) did pray between Maghrib and `Isha’. 
Ahmad (22926) narrated that Hudhayfah (may Allah be pleased with him) said: I came to the Prophet (peace and blessings of Allah be upon him) and prayed Maghrib with him. When he had finished the prayer, he stood and prayed, and he kept praying until he prayed `Isha’. (Classed as authentic by Al-Albani in Irwa’ Al-Ghalil, 470)
It is also proven from some of the Companions (may Allah be pleased with them) that they used to pray between Maghrib and `Isha’. Abu Dawud (1321) narrated that Anas ibn Malik (may Allah be pleased with him) said, concerning the verse “Their sides forsake their beds, to invoke their Lord in fear and hope, and they spend (in charity in Allah’s Cause) out of what We have bestowed on them” [As-Sajdah 32:16]: They used to wake up [according to another report: pray Nafl] between Maghrib and `Isha’ . Al-Hasan used to say: Qiyam Al-layl. (Classed as authentic by Al-Albani in Sahih Abu Dawud) 
Ibn Mardawayh narrated in his Tafsir that Anas (may Allah be pleased with him) said concerning this verse: They would pray between Maghrib and `Isha’. Al-`Iraqi said: Its chain of narrators is sound. (Quoted from `Awn Al-Ma`ud) 
Ash-Shawkani (may Allah have mercy on him) said in Nayl Al-Awtar (3/68): 
“The verses and Hadiths quoted concerning this matter indicate that it is prescribed to pray a great deal between Maghrib and `Isha’. Although most of the Hadiths are inauthentic, they carry some weight when taken all together, especially with regard to virtuous actions. Al-`Iraqi said: Among the Companions who used to pray between Maghrib and `Isha’ were `Abdullah ibn Mas`ud, `Abdullah ibn `Amr, Salman Al-Farisi, Ibn `Umar, Anas ibn Malik and some of the Ansar. Among the Successors were Al-Aswad ibn Yazid, Abu `Uthman An-Nahdi, Ibn Abu Malikah, Sa`id ibn Jubayr, Muhammad ibn Al-Munkadir, Abu Hatim, `Abdullah ibn Sakhbarah, `Ali ibn Al-Husayn, Abu `Abd Ar-Rahman Al-Habali, Shurayh Al-Qadi, `Abdullah ibn Maghfal and others. Among the imams there was Sufyan Ath-Thawri.” 
Based on this, it is recommended to pray between Maghrib and `Isha ’ without specifying a set number of Rak`ahs.  
For more details, please see the following answers: 175137 , 12844 , 203681 , and 38606 .
And Allah knows best.