The severity of withholding zakah

It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever leaves behind him a hoarded treasure (kanz: wealth on which zakah was not paid), on the Day of Resurrection it will be made to appear to him as an extremely venomous, bald serpent with two black spots above its eyes. It will pursue his mouth, and the person will say: Woe to you, what are you? It will reply: I am your treasure which you left behind you. It will keep pursuing him until he puts his hand into its mouth and it chews it up; then it will swallow the rest of his body as well."
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs did not record it. There is also an authentic supporting hadith (shahid) for it that meets his condition:

It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "On the Day of Resurrection, the treasure of one of you (the wealth on which zakah was not paid) will appear as a bald serpent with two black marks over its eyes. It will pursue its owner, and the owner will seek refuge from it, but it will keep following him while he flees from it, until it takes his finger into its mouth."
Imam Bukhari and Imam Muslim recorded the hadiths of Sayyiduna Ibn Mas'ud and Sayyiduna Ibn Umar (may Allah be pleased with them) in this chapter in abridged form (as a stern warning against withholding zakah), but they did not record this particular hadith of Sayyiduna Abu Hurayrah and Sayyiduna Thawban (may Allah be pleased with them both).

Sulaym bin Amir al-Kala'i states: I heard Sayyiduna Abu Umamah (may Allah be pleased with him) saying that during the Farewell Hajj the Messenger of Allah (peace and blessings be upon him) stood among us, mounted on his she-camel al-Jad'a. He (peace and blessings be upon him) had placed both his feet in the stirrups and was raising his voice so that all the people could hear. He (peace and blessings be upon him) asked: "Can you hear my voice?" A man from the crowd said: (Yes.) What do you command us? He (peace and blessings be upon him) said: "Worship your Lord, pray your five (daily) prayers, fast your month (of Ramadan), pay the zakah on your wealth and obey those in authority over you; you will enter the Paradise of your Lord." The narrator says: I asked: O Abu Umamah! How old were you on that day? He said: O my nephew! At that time I was thirty years old, and I was pushing forward between the camels (of the people), making my way to get close to the Messenger of Allah (peace and blessings be upon him).
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs did not record it.

Sayyiduna Abdullah bin Shaddad bin al-Had states: We went in to the Mother of the Believers Sayyidah Aishah (may Allah be pleased with her), and she told us that the Messenger of Allah (peace and blessings be upon him) came in to me and saw some thick silver bangles (sikhab) on my hand. He (peace and blessings be upon him) asked: "O Aishah! What is this?" I said: O Messenger of Allah! I had them made to adorn myself for you. He (peace and blessings be upon him) asked: "Do you pay the zakah on them?" I said: No (or whatever Allah willed). He (peace and blessings be upon him) said: "Then they are enough to take you to the Fire."
This hadith is sahih (authentic) on the condition of the two Shaykhs, but they did not record it.

Sayyidah Umm Salamah (may Allah be pleased with her) states that she used to wear gold ornaments (awdah). She asked the Prophet (peace and blessings be upon him) about this, whether it came under the ruling of "kanz" (the forbidden hoarded treasure about which a warning has come). He (peace and blessings be upon him) said: "Wealth whose zakah you have paid does not remain 'kanz' (hoarded treasure)."
This hadith is sahih (authentic) on the condition of Imam Bukhari, but the two Shaykhs did not record it.

It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When you have paid the zakah on your wealth, you have removed its evil from yourself."
This hadith is sahih (authentic) on the condition of Imam Muslim, but they did not record it, while there is also an authentic supporting hadith (shahid) for it from the narration of the people of Egypt: