Plucking the underarm hair is Mandoob by agreement of the ‘Ulamaa due to what was related from ‘Aa’ishah (ra) when she said: The Messenger of Allah صلى الله عليه وسلم said: “There are ten from the (acts of) Fitrah: cutting the moustache, plucking the armpit hair…” as recorded by Muslim and others.
In addition to what was related from Abu Hurairah (ra) from the Messenger of Allah صلى الله عليه وسلم who said: “The Fitrah are five: …and plucking the armpit hairs” as recorded by Muslim and others, and we quoted these two Hadeeth in full earlier.
And despite the wording in the Ahaadeeth being ‘plucking’ what is intended by this is the cleaning which could happen by shaving or cutting, which makes us permit both these acts and the timing of plucking the underarm hair in forty days is a Qareenah indicating the permission of this because the one who leaves his hair for forty days it would lengthen and reach the point where is can be shortened and cut (with scissors) and maybe even longer than that.
Younus Ibn Abdul ‘Aalaa said: “I approached Ash-Shaafi’ and he had somebody shaving his underarm and so Ash-Shaafi’ said: I learnt that the Sunnah is plucking but I cannot bare the pain.”
An-Nawawi: “The best is to pluck if you have the strength and it can also be done with shaving or liming” and Ibn Qudaamah agreed with him.
Ishaq was asked: “Is plucking more beloved by you or liming? He replied: Plucking if capable.”
So these Scholars preferred plucking over shaving and liming. Liming is the placing of lime upon the place of hair so that it falls off. And all of them permitted shaving and Al-Haff (rubbing) because the intended purpose is cleansing and it is achieved by all of these acts.
And it is recommended that the plucking or shaving is not delayed passed forty days due to the previously mentioned Hadeeth from Anas quoted in the issue related to trimming the moustache and if it is done in less time than this then this is permitted and perhaps it is better.
It is also Mandoob to start with the right underarm before the left due to what ‘Aa’ishah (ra) said: “The Nabi صلى الله عليه وسلم used to like to begin with his right whether it was related to putting on his shoes, walking and acts of purification and in everything” as recorded by Al-Bukhaari and Muslim.