The noble Sheikh
Muhammad bin Saalih Al-’Uthaimeen رحمه الله said in his explanation of forty hadeeths
of An-Nawawee رحمه الله: for acts of worship to be accepted-after
sincerity- they must conform with the way of the Prophet صلّى
اللَّهُ عليه وسلّم in six things:
1-The reason: it must be a one that is approved by
the Sharee’hah. For example if someone prayed the night prayer on the 27 of
Rajab or the 15 of Sha’baan. We say to him this act is rejected because Allah
did not make these two nights a reason to pray the night prayer.
2-The kind: for example if someone sacrificed a
horse on Eidul-Adhaa. We say this is rejected because the kinds
of sacrificial animals have been specified and horse is not one of
them.
3-The amount: for example if someone made ablution
four times on one limb or added a rak’ah to a prayer.