How to enjoin good and forbid evil to a person one does not know

Question:
If a person sees in someone something that contradicts the Shari'ah, and he does not know this person, and he fears that if he enjoins good upon him, he will not accept it, is it permissible for him to delay this advice until the change actually happens, by walking with him, so that he realizes his error and is convinced without him speaking to him? Or is it obligatory for him to enjoin good upon him as soon as he sees him in error?

Answer:
If a person sees, from his outward appearance, that a Muslim is in error, he should clarify it to him with good words and a good manner, and call him to Allah (Glorified and Exalted is He), and not delay it. He may not see him again, or he may die before that. The best of good is that which is immediate. Enjoining good and forbidding evil is an obligatory duty in its place, with no delay, except if he sees that he will not accept it from him and has already tried him. He must go to him with two or three of his good acquaintances; so that it is more acceptable for the call, and closer to its impact. This is another required thing.
As for if he fears missing the opportunity; he should forbid the evil in the good manner that he can use to have an effect on him, and ask his Lord for help in that, and pray for him for guidance, and not be harsh, and not be severe, but with the phrases that he thinks and hopes will be beneficial in forbidding the evil upon him, such as lagging behind in the prayers, such as his openly drinking intoxicants, such as his undutifulness to his parents, such as his severing his ties with his relatives, such as his shaving his beard, such as his letting his garments hang below the ankles, and the like of these evils that he sees openly.