Muhammad sinned, and I know that you're trying to twist and turn, eventually you'll say "Allah Knows Best", well other muslims who visit here will see this discussion so I'm going to continue.
Sahih al-Bukhari
Narrated Ibn 'Abbas: The Prophet used to invoke Allah at night, saying, "O Allah: All the Praises are for You: You are the Lord of the Heavens and the Earth. All the Praises are for You; You are the Maintainer of the Heaven and the Earth and whatever is in them. All the Praises are for You; You are the Light of the Heavens and the Earth. Your Word is the Truth, and Your Promise is the Truth, and the Meeting with You is the Truth, and Paradise is the Truth, and the (Hell) Fire is the Truth, and the Hour is the Truth. O Allah! I surrender myself to You, and I believe in You and I depend upon You, and I repent to You and with You (Your evidences) I stand against my opponents, and to you I leave the judgment (for those who refuse my message). O Allah! Forgive me my sins that I did in the past or will do in the future, and also the sins I did in secret or in public. You are my only God (Whom I worship) and there is no other God for me (i.e. I worship none but You)."
Muhammad beseeched Allah for the forgiveness of his sins in the past as well as those that he would commit in the future. He even mentioned his sins that he did in secret when nobody was looking.
When you pray, you ask for forgiveness of sins because you're a sinner.
48:2
That Allah may forgive thee of thy sin that which is past and that which is to come, and may perfect His favour unto thee, and may guide thee on a right path
He's a sinner, and again, why would Muhammad need to be guided on the straight path if he were not a sinner?
47:19
So know (O Muhammad) that there is no Allah save Allah, and ask forgiveness for thy sin and for believing men and believing women. Allah knoweth (both) your place of turmoil and your place of rest.
The above distinguishes between the sins of Muhammad and the sins of believers. Muhammad was instructed to request the forgiveness of his own sins as well as the sins of believers. Again this explicitly shows that Muhammad sinned and needed to ask forgiveness for his own evil deeds.