Praise be to Allaah.
Its Sunnah for a Muslim to say after every obligatory prayer, whether he prayed as an imam or behind an imam or on his own:
Astaghfirullaah (I ask Allaah for forgiveness) three times, then Allaahumma anta al-salaam wa minka al-salaam tabaarakta ya dhaa’l-jalaali wa’l-ikraam (O Allaah, 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:
Laa ilaaha ill-Allaah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer. Laa hawla wa laa quwwata illa Billaah wa laa na’budu illaa iyyaah. Lahu’l-ni’mah wa lahu’l-fadl wa lahu’l-thanaa’ al-hasan. Laa ilaaha ill-Allaah mukhliseena lahu’l-deena wa law kariha’l-kaafiroon. Allaahumma laa maani’ lima a’tayta wa laa mu’ti lima mana’ta wa la yanfa’ dhaa’l-jadd minka al-jadd (There is no god but Allaah 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 Allaah, 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 Allaah and we make our worship purely for Him (alone) however much the disbelievers may hate that. O Allaah, 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 Maghrib and Fajr prayer he should say, as well as the words mentioned above:
Laa ilaaha ill-Allaah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd yuhyi wa yumeet wa huwa ‘ala kulli shay’in qadeer (there is no God but Allaah 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 Subhaan Allaah wa’l-hamdu Lillaah wa Allaahu akbar (Glory be to Allaah, praise be to Allaah and Allaah is most great) 33 times, and to complete one hundred he should say:
Laa ilaaha ill-Allaah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer (There is no god but Allaah Alone, with no partner or associate His is the sovereignty and to Him be praise, and He is Able to do all things).
Its the Sunnah for the imam, the person who prays on his own and the person who prays behind an imam to recite these adhkaar(supplications) out loud after every obligatory prayer in a moderate voice that is not too loud, in an ordinary tone. It is proven in al-Saheehayn from Ibn ‘Abbaas (may Allaah 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 (blessings and peace of Allaah be upon him). Ibn ‘Abbaas (may Allaah be pleased with him) said: I would know that they had finished when I heard that.
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.
Then it is prescribed for the imam, the people praying behind an imam and the one who is praying alone to recite Aayat al-Kursiy silently, then for each of them to recite Qul Huwa Allaahu Ahad, Qul a’oodhu bi Rabbi’l-Falaq and Qul a’oodhu bi Rabbi’l-Naas [i.e., the last three soorahs of the Qur’aan] silently. After Maghrib and Fajr one should repeat Qul Huwa Allaahu Ahad, Qul a’oodhu bi Rabbi’l-Falaq and Qul a’oodhu bi Rabbi’l-Naas three times. This is the best because of the soundness of the reports that indicate all that we have stated above.
Allah knows best.