The Right of Allah and The True Happiness
All praises be to Allah, Lord of the universe, peace and prayer of Allah be upon the leader of righteous men, our prophet, Muhammad, his family, and all of his companions.
Verily, there are plenty of Allah’s rights that must be fulfilled by His slaves, which are everything that He has ordered His slaves to do.
The greatest among Allah’s rights, in general, is tawheed, i.e., believing in the Oneness of Allah. As explained in Shahih Al Bukhari and Muslim.
From Mu’adz -may Allah be pleased with him-, he said, “When I was riding behind the Messenger of Allah – peace and prayer of Allah be upon him- on a camel named ‘ufair, the Messenger of Allah -peace and prayer of Allah be upon him- said, “O Mu’adz! Do you know the right of Allah that must be fulfilled by His slaves, and the right of His slave that must be fulfilled by Allah?”. I replied, “Allah and His messenger knows better.” He said, “Verily, the right of Allah that must be fulfilled by His slave is to worship Him and not associating any partner with Him (committing shirk). And the right of the slave that must be fulfilled by Allah is that He will not torture anyone who doesn’t commit shirk to Him.”