A’maal (Deeds) are classified into three types.
(1) Beneficial deeds which are commanded by the Shariah. They are classified as Fardh, Waajib, Sunnat and Mustahab.
(2) Harmful deeds which are prohibited by the Shariah. These are known as Haraam and Makrooh.
(3) The third category consists of acts which are neither commanded nor prohibited by the Shariah. These actions are termed Mubah (permissible). With regard to this third category, the actions will remain permissible only as long as they do not become a medium or an introduction for sins. If, for example, conversation leads to forgetfulness of Thikr or to lies, slander, etc., then the talk will no longer be permissible.)
A verbal statement (or even an isolated practice) of Rasulullah (sallallahu alayhi wasallam) does not necessarily render the act a Sunnat. However, his constancy in practising an act renders it Sunnat. (The same applies to the acts of the vast majority of the Sahaabah. Thus, the 20 Raka’ts Taraaweeh Salaat with jama’at are Sunnatul Muakkadah, whereas the isolated act of planting a twig on the grave is not Sunnat because Rasulullah-sallallahu alayhi wasallam—did this act only once. He never instructed his Sahaabah to observe it, nor did he repeat it, nor did the Sahaabah adhere to it.)
Changing conditions or circumstances can give rise to a change in a ruling. In one instance something may be good whereas in a different situation it will be evil. There is no question in the goodness of Salaat. But it becomes Makrooh to perform Salaat when there is a strong urge to answer the call of nature(or if Salaat is performed during forbidden times or more raka’ts than ordained by the Shariah are performed, e.g. four raka’ts for Fajr instead of two.) Similarly, an act may have been permissible during a certain era because there were no factors of Karaahat(abomination) attached to the act, and later due to the Makrooh accretions the act became unlawful. For example, during the age of Rasulullah (sallallahualayhi wasallam) women were allowed to attend the Musjid. However, later the Sahaabah unanimaously prohibited them due to the malpractices which developed. During the age of Nabi-e-Kareem (sallallahu alayhiwasallam), the fitnah(moral corruption) which developed in later years did not exist.(The same argument is applicable to the prohibition of women attending walimah functions.)There are numerous such examples.