I heard that Al-Nawawi in his explanation of a hadith in Al-Bukhari 5241 said that it is haram for a man to touch another man except for handshaking. Is this true? Does that mean that if I touch another man's arm, I am sinning?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We are neither aware of this Hadeeth nor of the statement of An-Nawawi in this regard.

The numbering (of Ahadeeth) in Saheeh Al-Bukhaari varies according to the version of the copy.

In any case, it does not appear that this statement is true, or that a man touching another man's arm without desire is forbidden and that the doer is sinful. The evidence that this statement is not true is that a man standing side by side and sticking his shoulder to the man’s shoulder next to him during the congregational prayer is required. Additionally, it is permissible for a man to hug and kiss another man in certain cases.

Allah Knows best.