Sheikh Al-’Uthaymīn (may Allāh have mercy on him) said:

There are two conditions for an act of worship to be accepted:

1. It must be sincerely and exclusively for Allāh (Exalted be He)
2. It must be in accordance with the Sunnah (example/way) of the Messenger (May Allāh praise him and Peace be upon him)

The opposite of sincerity and being exclusively for Allāh is shirk (associating partners with Allāh). The opposite of being in accordance with the Sunnah is bid`ah (innovation) [Al-Sharḥ Al-Mumti’ ‘Alā Zād Al-Mustaqni’ Vol. 3 p.3].