(O ye who believe! Be not friendly with a folk with whom Allah is wroth…) [60:13].
This verse was revealed about some poor Muslims who used to inform the Jews of the news of Muslims and keep in touch with them. They got in exchange some of their fruits. And so Allah, glorified and exalted is He, warned them against this.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou