All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Prophet said to Mu‘adh Ibn Jabal : “Always say: ‘O Allah! Help me to remember You, thank You and worship You well’; after each prayer.” [Ahmad, Abu Dawood and al-Nassaa'i]
The scholars differed opinion concerning the meaning of “after each prayer” in this Hadeeth and the like.
Some scholars are of the view that “after each prayer” means after finishing the prayer by pronouncing Salam.
This opinion, without a doubt, has a favored position and it is the opinion of the majority of Muslim scholars.
Other scholars, including Ibn Taymiyah and Ibn al-Qayyim, believe that “after each prayer” means before finishing the prayer by pronouncing Salam.
This last opinion is strong since it is supported by the following dominant points:
1 - In most cases the phrase “after each prayer” means a portion that is part and parcel of something, i.e. a part of the prayer itself.
2 - Report of the Prophet's habitual actions after prayers are only confirmed to seeking forgiveness, saying “La Ilaaha Illa Allah” and “Glorification of Allah”. In other words, we have no narrations that include supplications after prayers directly.
3 - It is better to make Du'a (supplications) within the prayer than outside it.
Therefore, we say it is better to make Du'a before pronouncing Salam at the end of prayer. At the same time, one can make Du'a after finishing the prayer, but one is advised not to do that always since it is not reported from the Prophet . However, one can make Du'a after the stressed forms of Thikr (mention of Allah) that the Prophet was used to pronouncing them. Thus, it becomes clear that making Du'a for oneself, i.e. individually, does not constitute an innovation (Bid'ah).
As for Du'a in congregation, such a form is not reported in the Sunnah of the Prophet nor the practice of any of his companions. This means that the best guidance is that which is taken from the Prophet Muhammad .
He said: “Whoever performs an act which is not in accordance with ours [our religion], it will be rejected from him.” [Al-Bukhari and Muslim]
Allah knows best.