In both Shi`a and non-Shi`a sources, ʿAlī was given the epithet “Qā’id al-Ghurr al-Muḥajjalīn” by The Messenger of Allāh (sawa). It is difficult to dispute that this title given to him due to the abundance of narrations regarding this from both sides. An example of this that we find in the books of the ‘Ahlul-Sunnah’:
عن النبي (ص) أنه قال: أوحي إلي في علي ثلاث: إنه سيد المسلمين، وإمام المتقين وقائد الغر المحجلين
The Prophet (sawa) said: “It was revealed through revelation to me in regards to ʿAlī, three things: That he is the Master of the Muslims, the Imām of the pious, and the Qā’id al-Ghurr al-Muḥajjalīn.
‘Sunni’ Reference: al-Mustadrak ʿala al-Sahihain, al-Hākim, Volume 3, Page 138, Hadith #4668 (See Image below)
‘Sunni’ Grading: al-Hākim says it is Sahih, while al-Dhahabi did not concur.
Qā’id means commander. He is the one who gives orders to those who “drive” a thing, such as a passenger commanding the driver – or more specifically – a commander giving commands to his army. So ʿAlī is the Commander of al-Ghurr al-Muḥajjalīn.
As for al-Ghurr al-Muḥajjalīn (الغر المحجلين):
Ghurra is a speck of whiteness, and is often used to describe the beautiful white mark upon the forehead of a horse. A person who is aghar (أغر ) is a distinguished, eminent, or beautiful person, who – like a horse who is distinguished by his white marking – has a quality that makes them exceptionally remarkable in comparison to others.
On the other hand Ḥijl can also be translated as a sort of “whiteness”, and is often used to describe an ewe (female sheep) which is completely black, except for having white fore and hind shanks. Ḥijl is also used to refer to a thing that goes around the foot such as an anklet or shackle. In combining the two words, al-Ghurr al-Muḥajjal is commonly used to refer to a good highbred horse, that has white marks upon its forehead and feet which highlights its beauty and honourable origins.
In this context, it is said the believers will have these white marks (i.e. the light/nūr of faith and piety) upon their forehead, hands (or arms) and feet – which distinguish them, and their elevated ranks, with others. This will be a sign of beauty and honourable origins that others will yearn for. Thus, the commander of al-Ghurr al-Muḥajjalīn refers to ʿAlī being the commander of the believers, who will have the marks of whiteness upon their foreheads, hands, and feet. It is suggested that those parts of the body correlate directly with the places we perform ablution, and that our ablution is a means of “whitening” our foreheads, hands/arm, and feet, which we hope will be a marking that will distinguish us from others. This is in line with the supplication recommended during ablution while wiping our faces: ‘Oh Allāh! Whiten my face on the Day You whiten the faces and blacken the faces, and do not blacken my face on the Day you whiten the faces and blacken the faces’ (اللهم بيض وجهى يوم تبيض وجوه و تسود وجوه ولا تسود وجهى يوم تبيض وجوه وتسود وجوه).
Combining Qā’id with al-Ghurr al-Muḥajjalīn begins to paint a vivid picture, in particular to the Day of Judgement. While darkness envelops on the Day of Judgement, as all of creation approaches the Sirāt (the hair-narrow bridge to cross over to reach Paradise, see here for more info), the Shia of ʿAlī approach first. Just like a minor in dark mine – wearing a torch on helmet to guide him the way, the Shia of ʿAlī see clearly the path due to the light emanating from their foreheads, hands, and feet. Leading the way – like a commander – is none other than ʿAlī bin abi Talib. Following his footsteps and his command, charging towards salvation – like an army following their commander – are al-Ghurr al-Muḥajjalīn. Hence, the the Prophet (sawa) gave the epithet to ʿAlī (as) ~ Qā’id al-Ghurr al-Muḥajjalīn.