Have Taqwa of Allah and give up what remains from Riba (Surah Al-Baqarah 2: 278-279)

And if you do not do it, then take a notice of war from Allah and His Messenger but if you repent,you shall have your capital sums. Deal not unjustly, and you shall not be dealt with unjustly. (Surah Al-Baqarah 2: 279)


The Necessity of Taqwa and Avoiding Riba

Allah commands His believing servants to fear Him and warns them against what would bring them closer to His anger and drive them away from His pleasure. Allah said,

O you who believe! Have Taqwa of Allah  meaning, fear Him and remember that He is watching all that you do.

And give up what remains of Riba  meaning, abandon the Riba that people still owe you upon hearing this warning,

if you indeed have been Believers  believing in the trade that He allowed you and the prohibition of Riba. Zayd bin Aslam, Ibn Jurayj, Muqatil bin Hayyan and As-Suddi said that this Ayah was revealed about Bani `Amr bin `Umayr, a sub-tribe of Thaqif, and Bani Al-Mughirah, from the tribe of Bani Makhzum, between whom were outstanding transactions of Riba leftover from time of Jahiliyyah. When Islam came and both tribes became Muslims, Thaqif required Bani Al-Mughirah to pay the Riba of that transaction, but Bani Al-Mughirah said, "We do not pay Riba in Islam.'' `Attab bin Usayd, the Prophet's deputy on Makkah, wrote to the Messenger of Allah (Sallallahu Alayhi wa Sallam) about this matter. This Ayah was then revealed and the Messenger of Allah (Sallallahu Alayhi wa Sallam) conveyed it to `Attab,

O you who believe! Be afraid of Allah and give up what remains (due to you) from Riba (from now onward), if you are (really) believers. And if you do not do it, then take a notice of war from Allah and His Messenger.

They said, "We repent to Allah and abandon whatever is left of our Riba'', and they all abandoned it This Ayah serves as a stern threat to those who continue to deal in Riba after Allah revealed this warning.


Riba Constitutes War Against Allah and His Messenger

Ibn Jurayj said that Ibn `Abbas (Radhi Allahu Anhu) said that,

then take a notice of war means, "Be sure of a war from Allah and His Messenger.'' He also said, "On the Day of Resurrection, those who eat Riba will be told, `take up arms for war.''' He then recited,

And if you do not do it, then take a notice of war from Allah and His Messenger.  

`Ali bin Abi Talhah said that Ibn `Abbas (Radhi Allahu Anhu) said about,

And if you do not do it, then take a notice of war from Allah and His Messenger, "Whoever kept dealing with Riba and did not refrain from it, then the Muslim Leader should require him to repent. If he still did not refrain from Riba, the Muslim Leader should cut off his head.''
Allah then said,

But if you repent, you shall have your capital sums. Deal not unjustly by taking the Riba,

And you shall not be dealt with unjustly meaning, your original capital will not diminish. Rather, you will receive only what you lent without increase or decrease. Ibn Abi Hatim recorded that `Amr bin Al-Ahwas said, "The Messenger of Allah (Sallallahu Alayhi wa Sallam)  gave a speech during the Farewell Hajj saying;

Verily, every case of Riba from the Jahiliyyah is completely annulled. You will only take back your capital, without increase or decrease. The first Riba that I annul is the Riba of Al-`Abbas bin `Abdul-Muttalib, all of it is annulled.


Source: Tafsir ibn Kathir (Abridged)