Adhkar After Salah
Question

I would like to know the adkhar (words of remembrance) and du’as (supplications) that may be recited after the obligatory prayer.
Praise be to Allah.Adhkar after obligatory prayers
“The Sunnah (Prophetic guidance) is for the Muslim to say after every obligatory prayer , whether he prayed as an imam (leader) or behind an imam or on his own: 

Astaghfirullah (I ask Allah for forgiveness) three times, 
Then Allahumma anta al-salam wa minka al-salam tabarakta ya dhal-jalali wa’l-ikram (O Allah, You are the One Who is free from all defects and deficiencies and from You is all peace, blessed are You, O Possessor of majesty and honour). 
Then he should turn to face the people if he was the imam, then he and the other worshippers, and similarly the one who prayed on his own should say: La ilaha ill-Allah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadir. La hawla wa la quwwata illa Billah wa la na’budu illa iyyah. Lahu’l-ni’mah wa lahu’l-fadl wa lahu’l-thana al-hasan. La ilaha ill-Allah mukhlisina lahu’l-dina wa law kariha’l-kafirun. Allahumma la mani’a lima a’tayta wa la mu’tiya lima mana’ta wa la yanfa’u dha’l-jaddi minka al-jadd  (There is no god but Allah Alone, with no partner or associate His is the sovereignty and to Him be praise, and He is Able to do all things. There is no power and no strength except with Allah, and we worship none but Him.  From Him (Alone) come all blessings and favours, and all good praise is due to Him. There is no god but Allah and we make our worship purely for Him (alone) however much the disbelievers may hate that. O Allah, none can withhold what You give and none can give what You withhold, and no wealth or majesty can benefit anyone for from You is all wealth and majesty).
After the Maghrib (sunset) and Fajr (dawn) prayers he should say, as well as the words mentioned above: La ilaha ill-Allah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd yuhyi wa yumit wa huwa ‘ala kulli shay’in qadir (there is no God but Allah Alone, with no partner or associate, His is the Dominion and to Him be praise, He gives life and gives death, and He has power over all things) ten times. Then after that he should say Subhan Allah wa’l-hamdu Lillah wa Allahu akbar (Glory be to Allah, praise be to Allah and Allah is most great) 33 times, and to complete one hundred he should say: La ilaha ill-Allah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shayin qadir (There is no god but Allah Alone, with no partner or associate His is the sovereignty and to Him be praise, and He is Able to do all things). 
Then it is prescribed for the imam, the people praying behind an imam and the one who is praying alone to recite Ayat al-Kursiy silently, then for each of them to recite Qul Huwa Allahu Ahad , Qul a'udhu bi Rabbi’l-Falaq and Qul a’udhu bi Rabbi’l-Nas [i.e., the last three surahs of the Quran] silently.

After Maghrib and Fajr one should repeat Qul Huwa Allahu Ahad, Qul a'udhu bi Rabbi’l-Falaq and Qul a’udhu bi Rabbi’l-Nas three times. This is the best because of the soundness of the reports that indicate all that we have stated above.
How to say adhkar after Salah
The Sunnah is for the imam, the person who prays on his own and the person who prays behind an imam to recite these adhkar out loud after every obligatory prayer in a moderate voice that is not too loud, in an ordinary tone. 
It is proven in al-Sahihayn from Ibn ‘Abbas (may Allah be pleased with him) that raising the voice in dhikr when the people have finished the obligatory prayer was what was done at the time of the Prophet (peace and blessings of Allah be upon him). Ibn ‘Abbas (may Allah be pleased with him) said: I would know that they had finished when I heard that. 
Is dhikr in unison after prayer allowed?
It is not permissible to recite out loud in unison; rather each person should recite to himself without paying attention to the voice of anyone else, because reciting dhikr in unison is an innovation for which there is no basis in Islam. 
May Allah send peace and blessings upon our Prophet Muhammad and his family and Companions and those who followed them in truth until the Day of Judgement.”
And Allah knows best.