Not washing the body parts properly while making Wudhu is a Major Sin.

بسم الله والحمد لله والصلاة والسلام على رسول الله ، وبعد

From Abu Hurairah, `Abdullaah Ibn `Amr Ibn al-`Aas, and `Aaishah رضي الله عنهم that the Prophet ﷺ said:

❝Woe to the heels because of Hell-fire.❞

[Saheeh al-Bukhaaree and Saheeh Muslim. In the narration of Saheeh Muslim it is reported that Allaah’s Apostle ﷺ saw a man who did not wash his heel so he ﷺ said the above]

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Shaikh Muhammad Ibn Saaleh al-`Uthaymeen رحمه الله said (paraphrased): 

۞ The “heels” here are in reference to those heels which have not been washed properly while making Wudhu (ablution).

۞ (The stern warning) is because Tahaarah (purification) is one of the most important conditions of the Salaah (prayer), and any imperfection (or deficiency) in it is in fact a deficiency in the Salaah (which will nullify the Salaah).

۞ (In this Hadeeth) the Prophet ﷺ warned against failing to do the Wudhu properly; wherein he threatened the one who did not perform his Wudhu properly - by not washing the body parts which are meant to be washed while making Wudhu – with the punishment of the Hell-fire on that missed spot (i.e. the meaning of this Hadeeth is not limited to the heels only. Also included in this would be a person who might not wash his elbows properly while washing his forearms etc.)

۞ The heels were specifically mentioned in this Hadeeth because this was the spot that was neglected at that instant of time which led to the Prophet ﷺ to say that statement.

۞ Failing to wash the body parts properly – which are meant to be washed while making Wudhu – is considered as a Major sin (due to the threat mentioned in this Hadeeth).

۞ Washing the feet while making Wudhu is Waajib (obligatory) if they are uncovered (i.e. when one is not wearing socks  or shoes over them).

[Tanbeeh al-Afhaam Sharh `Umdatil Ahkaam of Shaikh Ibn al-`Uthymeen, Hadeeth #3]