Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa
1- It is permissible to greet another person after Prayer or making duaa by saying, for example, taqabbal Allah, (may Allah accept from you).
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states;
Saying Taqabbala Allah after prayer
If anyone thinks that saying, ‘Taqabbala Allah’ (may Allah accept your prayer) after Salat al Jamaah (congregational prayer) is a necessary part of prayer, believing that prayer is incomplete without it, then it is certainly akin to instituting bidah (innovation).
Thus, we would say there is nothing wrong with shaking hands and greeting one another after prayer. This ruling applies if we do it with the intention that one wishes his brother’s prayer to be accepted.
This is because of the fact that religion is what Allah and His Messenger prescribed. Since no such ritual has been prescribed by them, it shall be bidah.
On the contrary, if what we said above is not the case, then we cannot consider it bidah.
If someone tells you “may Allah accept our Prayer”, how can it be bid`ah? If we apply this logic, then saying good words, or acting kindly towards one another become bidah, on the pretext that the Prophet (PBUH) and his Companions had never done or said so in the precise manner at the precise time and place. Most certainly that is not the intent and purpose of interdiction against innovations.
If, therefore, saying, ‘Taqaballa Allah’ is an innocent act, replying to it, however, is indeed a most desirable act. We should reciprocate goodness with what is better or at least with a similar one. Allah Almighty says, ‘If you are greeted with a greeting respond with a better greeting or at least a similar one.’ (An-Nisaa 4:86)
However, if one is making duaa, it is a matter of preference to wait until he finishes the duaa and then shake hands with him or say, ‘taqaballa Allah’.
Allah Almighty knows best.