The status of the hadeeth, “If I am someone’s mawla then ‘Ali is his mawla too” and its meaning
Question

                How sound is the hadeeth, “If I am someone’s mawla (friend) then ‘Ali is his mawla too”, and what does it mean?
Praise be to Allah.
    This hadeeth was narrated by al-Tirmidhi, 3713; Ibn Maajah, 
    121. There is some difference of opinion as to its authenticity. Al-Zayla’i 
    said in Taareekh al-Hidaayah 1/189: “How many ahaadeeth there are 
    which have many narrators and many isnaads, but they are da’eef (weak), such 
    as the hadeeth “If I am someone’s mawla then ‘Ali is his mawla too”.” 

    Shaykh al-Islam Ibn Taymiyah said: “As for his saying “If I 
    am someone’s mawla then ‘Ali is his mawla too”, this is not in the books of
    Saheeh, but it is one of the reports which were narrated by the 
    scholars and concerning whose authenticity the people disputed. It was 
    narrated that al-Bukhaari, Ibraaheem al-Harbi and a group of scholars of 
    hadeeth stated that it is not saheeh… As for the additional material, which 
    is the phrase ‘O Allaah, take as friends those who take him as a friend, and 
    take as enemies those who take him as an enemy,’ etc., this is undoubtedly 
    false.” Manhaaj al-Sunnah, 7/319. Al-Dhahabi said: “As for the 
    hadeeth, “If I am someone’s mawla then ‘Ali is his mawla too”, it has jayyid 
    isnaads.” It was classed as saheeh by al-Albaani in al-Silsilah 
    al-Saheehah, 1750, and he criticized those who said that it is da’eef 
    (weak). 

    The fact that this sentence has a saheeh isnaad going back to 
    the Prophet  (peace and blessings of Allaah be upon him) – if it is 
    saheeh – cannot under any circumstances be taken as evidence to support what 
    the extremists added to the hadeeth to prove that ‘Ali (may Allaah be 
    pleased with him) takes precedence over all the other Sahaabah, or to 
    slander the Sahaabah and accuse them of usurping his rights. Shaykh al-Islam 
    [Ibn Taymiyah] referred to some of these additions and proved them to be 
    da’eef (weak) in ten places in Manhaaj al-Sunnah. 

    There is some difference of opinion as to the meaning of the 
    hadeeth. Whatever the meaning, it does not contradict that which is proven 
    and well-known from the saheeh ahaadeeth which state that the best of this 
    ummah is Abu Bakr and that he was the most deserving of being the khaleefah; 
    then he was followed by ‘Umar, then ‘Uthmaan – may Allaah be pleased with 
    them all. Proof of the virtue of a specific Sahaabi does not means that he 
    is the best of them, and that does not contradict the fact that Abu Bakr is 
    the best of them as is affirmed in the chapters on ‘aqeedah. 

    One of the meanings that have been suggested for this hadeeth 
    is as follows: 

    “It was said that its meaning is, ‘whomever I took as a 
    friend, ‘Ali will also take him as a friend as opposed to an enemy, and 
    whomever I used to love, ‘Ali will also love him.’ And it was said that its 
    meaning is, ‘whoever took me as a friend, ‘Ali will also take him as a 
    friend.’ This was quoted by al-Qaari’ from some of his scholars. Al-Jazari 
    said in al-Nihaayah: ‘The word mawla is frequently mentioned in the 
    hadeeth, and this is a name that is applied to many. It may refer to a lord, 
    to an owner, to a master, to a benefactor, to one who frees a slave, to a 
    supporter, to one who loves another, to a follower, to a neighbour, to a 
    cousin (son of paternal uncle), to an ally, to an in-law, to a slave, to a 
    freed slave, to one to whom one has done a favour. Most of these meanings 
    are referred to in various ahaadeeth, so it is to be understand in the 
    manner implied by the context of the hadeeth in which it is mentioned. 
    Everyone who is in charge of some matter or is taking care of it is the 
    mawla of that thing. The word mawla mentioned in this hadeeth may refer to 
    most of the meanings indicated above. Al-Shaafa’i (may Allaah have mercy on 
    him) said: What is meant by that is the bonds of Islam, as in the aayah 
    (interpretation of the meaning): 

    ‘That is because Allaah is the Mawlaa (Lord, Master, 
    Helper, Protector) of those who believe, and the disbelievers have no Mawlaa 
    (lord, master, helper, protector)’

    [Muhammad 47:11]
Al-Teebi said: it is 
    incorrect to interpret the mawla as referring to the imam who conducts the 
    affairs of the believers, because the only person who was in charge of the 
    Muslims’ affairs during the lifetime of the Prophet  (peace and 
    blessings of Allaah be upon him) was the Prophet himself and no one else, so 
    the word mawla must be interpreted as referring to love, the bonds of Islam 
    and so on.” 
Adapted from Tuhfat al-Ahwadhi Sharh 
    al-Tirmidhi, Hadeeth 3713..