In Bani Thaqif tribe, there was an experienced physician named Harith bin Kaldah Thaqafi.
He came to the Holy Prophet (s.a.w.s.) and said: O Muhammad!
I have come to treat you (God forbid) for insanity as I am expert in this matter and many lunatics have recovered through my treatment.
The Holy Prophet (s.a.w.s.) said: How is it that you consider me a lunatic while you yourself are acting like mad people?
You have not yet either examined me nor have you still thought at all about my truthfulness or otherwise?!
Harith replied: Have I not yet known your truthfulness or telling lies when you claim to be a messenger but are unable to show your ability about it?
The Holy Prophet (s.a.w.s.) said: These words of yours are a deed of a lunatic, because before uttering, you have not asked me why I make a claim and you have not found me unable to produce any proof required by you.
Harith replied: It is true.
So now I demand from you a miracle to try you. If you are a messenger of Allah, call up this tree (pointing towards a deeply rooted big tree). If it comes to you, I will know that you are God’s prophet. I will testify to it.
Otherwise I will repeat that you are a lunatic as I have heard.
The Holy Prophet (s.a.w.s.) pointed his holy hand towards that tree, calling it to him.
At once that tree got itself uprooted and rushed speedily towards the Holy Prophet (s.a.w.s.) digging the ground like a canal and then stood meekly in front of the Holy Prophet (s.a.w.s.).
It spoke most clearly: O Allah’s Messenger! Here I am. Kindly give command!
The Holy Prophet (s.a.w.s.) told that tree: I have called you to testify that the God is Only One Allah and that I am Allah’s messenger, and thereafter, that Ali (a.s.) is the Imam.
You should also testify that Ali (a.s.) is my confidant, assistant and helper, of whom I am proud and that had he not been born, Allah would not have created any of His creations.
That tree at once spoke up:
I give witness that there is no one worth worship, except Only One Allah; that He has no partner. I testify that, O Muhammad (s.a.w.s.)! You are His servant and His Messenger.
He has sent you with truth, so that you may give glad tidings of Paradise to the obedient people and warn the defiant, disbelievers and sinners of Hellfire.
You may invite Allah’s creations towards Him and you may be a Lamp of Guidance for all.
I also testify that Ali Ibne Abi Talib (a.s.), your cousin, is your brother-in-faith who makes maximum effort in the matter of religion and Islam and that he is your confidant, supporter and uprooter of your enemies and helper of your friends and the door of your knowledge.
I also testify that all of your friends who befriend him and who regard his enemies as their own foes will enter Paradise and also I testify that your enemies who befriend your enemies and who are enemies of your friends will fill Hell.
Thereafter, addressing Harith, the Holy Prophet (s.a.w.s.) asked: O Harith! Can one who produces such miracles ever be a lunatic?!
Harith then said: O Allah’s Messenger! By Allah, it can never be so.
Therefore, I give witness that you are the messenger of the Lord of the Universe and the chief of the entire creation.
His Islam was thus nice.
[Source: Tafseer of Imam Hasan Askari(as), Sura Baqara, Verse 23-25]