The Qur'an says: And We have made every man's bird of omen to cling to his neck, and We will bring forth to him on the resurrection day a book which he will find wide open (17:13). What does it mean?
Al-Mufradat says: One's "bird of omen" is his actions, whether good or evil, which "flies" from him.
Al-Mizan says that it means what indicates good or bad fortune or rather it reveals a good or bad outcome. It further says:
Every person has something linked to his ultimate fate that would indicate whether it will be good or bad. Making it cling to a person's neck means making one's action inseparable from him. The neck is part of the body which cannot be separated from the person. The good or bad actions of a person do not leave him for anyone else, but they return to him, by a decree from God.
God's definitive decree is that good fate is a result of faith and God-fearing and bad fate is a result of disbelief and disobedience. This can be deduced from: And surely Hell is the promised place of them all...Surely those who are God-fearing shall be in the midst of gardens and fountains (15:43-45).
It necessitates that one's actions that determine his fate are attached to him and it is a binding attachment that can never be severed. It has been decreed that every action belongs to its doer. The fate of obedience is paradise and the fate of disobedience is hell.
Based on the above, the verse (17:13) is saying that one's ultimate happiness or misery hinges only upon the good or bad deeds that he earns through choice or free will. This is exactly the opposite of the false claim by some exegetes that one's happiness or wretchedness is predestined, irrespective of one's actions and obedience.
Reference: Al-Mizan English Volume 25, pp. 69-71, first edition, Tawheed Institute Australia Ltd., 2015.