The Necessity of Taqwa and Avoiding Riba

O you who believe! Have Taqwa of Allah and give up what remains from Riba, if you are (really) believers. 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. (2:278-279)
Tafsir (Ibn Kathir)
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 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 about this matter. This Ayah was then revealed and the Messenger of Allah 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 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.)

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.