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 allowed to greet another person after Prayer or making du`a’ for saying, for example, taqabbal Allah, (may Allah accept from you).
In response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
If anyone thinks that saying, ‘Taqabbala Allah’ (may Allah accept your prayer) after Salatul Jama`ah (congregational prayer) is a necessary part of Salah, believing that the Salah is incomplete without it, then it is certainly akin to instituting bid`ah (innovation).
Thus, we would say there is nothing wrong with shaking hands and greeting one another after prayer if it is done with the intention that one wishes his brother’s prayer to be accepted.
This is because of the fact that religion is what is prescribed by Allah and His Messenger, and since no such ritual has been prescribed by them, it shall be considered as bid`ah.
If, on the contrary, what we said above is not the case, i.e., if it happens that one says these words with the intention of wishing that his brother’s prayer be accepted, and hoping that his brother also reciprocates in the same way for his prayer also to be accepted, then we cannot consider it bid`ah.
If someone tells you “may Allah accept our Prayer”, how can it be considered bid`ah? If we apply this logic, then saying good words, or acting kindly towards one another become bid`ah, on the pretext that the Prophet (peace and blessings be upon him) 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 considered an innocent act, replying to it, however, is indeed a most desirable act, for we are to 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-Nisa’ 4:86)
However, if one is making du`a’, it is a matter of preference to wait until he finishes the du`a’ and then shake hands with him or say, ‘taqaballa Allah’.
Allah Almighty knows best.
Excerpted with slight modifications from, www.muslims.ca