Meaning of ‘Ali رضي الله عنه being a Mawla

A group came to ʿAlī at al-Raḥbah and said: “Al-Salāmu ʿalayka, O our Mawlā.” He said: “How can I be your Mawlā when you are an Arab people?!” They said: “We heard the Messenger of Allāh ﷺ saying on the Day of Ghadīr Khumm: ‘Whomever I am his Mawlā, then this (man) is his Mawlā.'” Riyāḥ said: So when they left, I followed them and asked: “Who are these?” They said: “A group from the Anṣār, among them was Abū Ayyūb al-Anṣārī.” [Musnad Aḥmad (23563); Isnād Ṣaḥīḥ]

Some more references for the hadith include al-Jami’ by Ma’mar (20388), Hadith ‘Ali bin Hujr (471), Dhayl Tarikh Baghdad by Ibn al-Najjar (18/10), tafsir ibn al-Mundhir (2/679), Juz fihi ma intaqa Ibn Marduyah (105)

الحسن بن الحسن، وسأله، رجل، ألم يقل رسول الله صلى الله عليه وسلم: «من كنت مولاه فعلي مولاه» ، قال لي: بلى والله لو يعني بذلك رسول الله صلى الله عليه وسلم الإمارة ‌والسلطان ‌لأفصح لهم بذلك، فإن رسول الله صلى الله عليه وسلم كان أنصح للمسلمين، فقال: «يا أيها الناس، هذا ولي أمركم والقائم عليكم من بعدي فاسمعوا له وأطيعوا والله لئن كان الله ورسوله اختار عليا لهذا الأمر وجعله القائم به للمسلمين من بعده ثم ترك علي ما أمر الله ورسوله لكان علي أول من ترك أمر الله ورسوله
[الاعتقاد للبيهقي (ص ٣٥٥) وسنده حسن وشيخه هو المزكي]

Al-Ḥasan bin al-Ḥasan (al-Basri) was asked by a man: “Did not the Messenger of Allāh ﷺ say: ‘Whomever I am his Mawlā, then ʿAlī is his Mawlā’?” He said to me: “Yes, but by Allāh, if the Messenger of Allāh ﷺ meant by that the rulership and the authority, he would have been explicit to them about that. For indeed, the Messenger of Allāh ﷺ was the most sincere in advising the Muslims. He would have said: ‘O people, this is the Walī of your affair and the one in charge of you after me, so listen to him and obey.’ By Allāh, if Allāh and His Messenger had chosen ʿAlī for this matter and made him the one in charge of it for the Muslims after him, and then ʿAlī abandoned what Allāh and His Messenger commanded, then ʿAlī would be the first to abandon the command of Allāh and His Messenger.” [Al-Iʿtiqād by al-Bayhaqī (p. 355); Isnād Ḥasan]

Muḥammad bin Ziyād ibn al-Aʿrābī said: “Mawlā is the Owner,  which is Allāh. And Mawlā is the cousin, the emancipator, the emancipated, the neighbor, the partner, the ally, the lover, the follower, and Mawlā is the Walī (ally/close friend) and from this is the saying of the Prophet ﷺ: ‘Whomever I am his Mawlā, then ʿAlī is his Mawlā,’ meaning: ‘Whoever takes me as a Walī, let him take ʿAlī as a Walī.'”

Thaʿlab said: “And it is not as the Rāfiḍah say, that ʿAlī is the Mawlā (Owner) of the creation and their Master. The Rāfiḍah have committed Kufr in this, because it is invalidated by logic; for we saw him buying and selling. If all things were his property, then from whom would he buy and sell? Rather, it is from the perspective of love and obedience, and it indicates that the Mawlā and the Walī is the lover/supporter.” [Tārīkh Dimashq (42/238); Isnād Ḥasan]