The religion is what is prescribed by Allah and His Messenger.
Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
“If anyone thinks that saying: ‘Taqabala Allah’ (may Allah accept your prayer) after ‘Salatul Jama`ah‘ (congregational prayer) is a necessary part of Prayer, believing that it is incomplete without it, then it is certainly akin to instituting bid`ah (innovation). As the religion is what is prescribed by Allah and His Messenger (peace be upon him), 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 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: ‘taqabala 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 says:
If you are greeted with a greeting respond with a better greeting or at least a similar one. (An-Nisaa’ 4: 86)
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. However, if one is making du`aa’ (supplication), it is a matter of preference to wait until he finishes the du`aa’ and then shake hands with him or say, ‘taqabala Allah’.
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Source: onislam.net.