Say: "I do not ask of you a wage for this, except love for(my) relatives." (ash-Shura, 42:23 )
Here there is a need to ask why the rest of the prophets looked for no wage but the most noble Prophet asked for one for his message; why did he want friendship for his near relatives as a requital for his message?
The Qur'an itself provides an answer to this question:
That is to say, that which I ask for as a wage accrues to you, not to me; this friendship is a halter for your own perfection and reformation, and it is called a wage. Otherwise it is in fact another good which I recommend to you from the point of view that the Household and relatives of the Prophet are people who do not gather round defilement, and whose hems are clean and pure.
Love and devotion to these people brings no other result apart from obedience to the truth and adherence to virtues, and it is friendship for them which transmutes and perfects like the elixir.
Whatever the meaning of "relatives" may be, it is certain that the most obvious person to whom it is applicable is 'Ali. Imam Fakhru'd-Din ar-Razi says:
"Zamakhshari relates in his (Qur'anic exegesis) al-Kashshaf:`When this verse was sent down they said: "O Messenger of Allah! Who are the relatives to whom our love is due?" He said: " `Ali and Fatimah and their sons." '
"It is thus established from this tradition that these four persons are "relatives" of the Prophet, and that they should enjoy the respect and love of the people, and this matter can be reasoned out in a number of ways:
" 1 - The verse: except love for my relatives.
" 2 - There is no doubt that the Prophet dearly loved Fatimah, and he said: `Fatimah is a part of my body; what harms her harms me.' he also loved 'Ali and the Hasanayn (Hasan and Husayn), since a great number of mutawatir traditions (those which are narrated by so many as to make doubt impossible) have reached us on this subject. Thus friendship of them is obligatory on all the community,  because the Qur'an commands:
And follow him (the Prophet), haply you will be guided. (al-A'raf, 7:158)
"It also commands:
You have a good example in Allah's Messenger. (al-Ahzab, 33:21)
"These (considerations) prove that love for the Family of Muhammad - who are `Ali, Fatimah and the Hasanayn -is obligatory on all Muslims. " 
There are also many traditions from the Prophet concerning love and friendship for `Ali:
1: Ibn al-Athir narrates that the Prophet spoke to `Ali and said: "O `Ali, God has embellished you with things, no dearer embellishment than which exists before his slaves: resignation from the world has been ;appointed for you in such a way that neither do you profit from the world, nor it from you. On you has been bestowed the love of the wretched; they are proud of your leadership, and you also of their following you. Content is he who loves you, and is a true friend to you. And woe betide he who shows enmity towards you, and lies about you." 
2: as-Suyuti relates that the Prophet said: "Love of `Ali is faith, and enmity towards him is sedition." 
3: Abu Na'im narrates that the Prophet addressed the Ansar and said: "Shall I guide you to something which, if you grasp it after me, you will never go astray?" They said: "Yes, O Messenger of Allah ! " He said: "It is 'Ali: love him with the love (you have) for me, and respect him with the respect (you have) for me. For God has ordered me through Gabriel to tell you this." 
The Sunnis have also narrated traditions from the Prophet in which observing `Ali's face and talking of his virtues is counted as a form of worship.
1: Muhibb at-Tabari narrates from `A'ishah that she said: "I saw my father (Abu Bakr) gazing often at 'Ali's face. I said: `O my father! I see you gazing often at 'Ali's face.' He said: `O my daughter! I heard the Prophet say: "Looking at the face of `Ali is worship."' 
2: Ibn Hajar narrates from `A'ishah that the Prophet said: "The best of my brothers is `Ali, the best of my paternal uncles is Hamzah, and remembrance of 'Ali and speaking about him is worship." 
'Ali was the most loved person before God and the Prophet, and thus naturally the best of those who are loved. Anas ibn Malik says: "Every day, one of the children of the Ansar would do some task for the Prophet. One day my turn came. Umm Ayman brought a chicken dish before the Prophet and said: `Messenger of Allah! I have caught this chicken myself and cooked it for you.' He said: `O God! Send the best of (Thy) slaves that he may share with me in eating this chicken.' At that very moment someone knocked on the door and the Prophet said to me: `Anas! Open the door.' I said: `May God make it a man of the Ansar!' But I found 'Ali in front of the door, and I said: `The Prophet is busy.' Then I returned to stand in my place. Again there was a knock at the door, and the Prophet said: `Open the door.' Again I prayed that it would be someone from the Ansar. I opened the door and again it was 'Ali. I said: `The Prophet is busy.' Then I returned to stand in my place. Yet again there was a knock at the door, and the Prophet said: `Anas, go and open the door, and bring him in. You are not the first person to love your own people; that is not one of the Ansar.' I went and brought 'Ali in, and he ate the chicken dish with the Prophet." 
. The love of the Prophet towards them had no personal aspect, that is, it was not only because, for example, they were his children and grandchildren and if someone else had been in their place he would have loved them. The Prophet loved them because they were exemplary persons and God loved them, for the Prophet had other children whom he did not love to this extent and to whom his community had no such obligation.
. at- Tafsiru 'l-kabir, vo1.27, p.166 (Egyptian ed.).
. Usdu 'l-ghabah, vo1.4, p.23
. Kanzu 'l-`ummal. In as-Suyuti, Jam`u 'l-jawami `, vol.6, p.156.
. Hilyatu 'l-awliya', vol.1, p.63. There are many traditions on this subject, and we have come across more than ninety in authoritative Sunni texts, all of which concerns love for Amir al-mu'minin. There also exist many traditions in Shi'ah texts, and the respected scholar al-Majlisi has gathered them together in vol.39 (of the new edition) of Biharu 'l-anwar in the chapter on love and hatred for Amir al-mu'minin; he related 123 traditions in this chapter.
. ar-Riyadu'n-nadirah, vo1.2, p.219; and about another twenty traditions, as far as we are aware, have been related in Sunni texts on this subject.
. as-Sawa'iqu 'l-muhriqah, p.74; and five more traditions have been related in Sunni texts on this subject.
. al-Mustadrak `ala as-Sahihayn, vo1.3, p.131. This story is related in various ways in more than eighty narrations in authoritative Sunni texts.