Is it permissible to address Prophet Muhammad (peace and blessings be upon him) secretly or openly, and does he hear what is said to him, and is it permissible to say, "I am sorry, O Messenger of Allah," upon committing a sin, as an apology and not for repentance?

Whoever commits a sin is obligatorily required to repent to Allah the Almighty and apologize to Him. Repentance is due only to Allah alone, as He says: "And turn to Allah in repentance, all of you, O believers, that you might succeed." As for apologizing to the Prophet by saying, "I am sorry, O Messenger of Allah," this is not prescribed, as there is no evidence for it. Rather, what is prescribed when committing a sin is repentance and seeking forgiveness, for Allah the Almighty says: "And whoever does a wrong or wrongs himself but then seeks forgiveness of Allah will find Allah Forgiving and Merciful." Nor is there any authentic religious proof that Prophets hear such words in their graves. While we believe that they are alive in their graves and pray, the reality of this barzakh (interim) life is known only to Allah the Almighty. However, what has been reported is that if one sends blessings upon the Prophet, may Allah bless him and grant him peace, Allah returns his soul to him so that he may return the greeting.