Surah Al A'raf Tafseer

وَإِخْوَانُهُمْ يَمُدُّونَهُمْ إِنَّ الْمُبَذرِينَ كَانُواْ إِخْوَنَ الشَّيَـطِينِ يَمُدُّونَهُمْ فِي الْغَيِّ ثُمَّ لَا يُقْصِرُونَ

and they never stop short.

for the devils never cease inciting mankind to commit errors.

Ali bin Abi Talhah reported that Ibn Abbas commented on Allah's statement,

وَإِخْوَانُهُمْ يَمُدُّونَهُمْ فِي الْغَيِّ ثُمَّ لَا يُقْصِرُونَ

(But (as for) their brothers they plunge them deeper into error, and they never stop short),

"Neither mankind stop short of the evil that they are doing nor the devils stop short of luring them."

Therefore,

لَا يُقْصِرُونَ

(they never stop short),

refers to the devils getting tired or stopping their whispering.

Allah said in another Ayah,

أَلَمْ تَرَ أَنَّأ أَرْسَلْنَا الشَّيَـطِينَ عَلَى الْكَـفِرِينَ تَوُزُّهُمْ أَزّاً

See you not that We have sent Shayatin against the disbelievers to push them to do evil! (19:83)

persistently luring the disbelievers to commit evil, according to Ibn Abbas and others