Commas

In the following sentence , do I need to place a comma after the introductory participial phrase?

Walking in the street, the boy spotted one of his friends. 

I have read a grammar book, which mentions that you only need to separate the introductory phrase or clause from the sentence if it is adverbial, but I have also read another source on the internet which says that you must place a comma after an introductory phrase containing a participle.