From Buraydah, he said: “I participated in a military expedition with ʿAlī to Yemen and witnessed harsh treatment from him. When I arrived in the presence of the Messenger of Allāh ﷺ, I mentioned ʿAlī and criticized him. I saw the face of the Messenger of Allāh ﷺ change, and he said: ‘O Buraydah! Am I not more deserving of being kind to the believers than to criticize them?’ I said: ‘Yes, O Messenger of Allāh!’ He said: ‘For whomever I am his Mawlā, then ʿAlī is his Mawlā.’” [Al-Durr al-Manthūr]
Al-Shāfiʿī said: This means loyalty to Islām, as in His saying: “That is because Allāh is the protector of those who have believed and because the disbelievers have no protector.”
Abū al-ʿAbbās said regarding the statement of the Prophet ﷺ: “For whomever I am his Mawlā, then ʿAlī is his Mawlā,” meaning whoever loves me and follows me should follow him.
Al-muwālāh (loyalty) is the opposite of enmity, and al-walī (ally) is the opposite of enemy.
A Rāfiḍī said to him Al-Ḥasan ibn al-Ḥasan ibn al-Ḥasan ibn ʿAlī ibn Abī Ṭālib: “Did not the Messenger of Allāh ﷺ say to ʿAlī, ‘If I am the Mawlā of someone, then ʿAlī is his Mawlā’?”
He (al-Ḥasan) replied: “By Allāh, if the Prophet ﷺ had intended by that rulership, he would have been more explicit with you in conveying it, just as he was explicit with you regarding the Ṣalāh, Zakāh, and Ḥajj. He would have said, ‘O people! This ʿAlī is your leader after me.’
The Messenger of Allāh ﷺ gave the best counsel to the people, meaning he was clear in his message.
If it were as you claim, that ʿAlī had been appointed to this position after the Prophet ﷺ, then he would have been the most at fault among all the people for failing to act upon what the Prophet ﷺ commanded.” [Saḥīḥ]