وَمَاذَا عَلَيْهِمْ لَوْ ءَامَنُوا۟ بِٱللَّهِ وَٱلْيَوْمِ ٱلْـَٔاخِرِ وَأَنفَقُوا۟ مِمَّا رَزَقَهُمُ ٱللَّهُ ۚ وَكَانَ ٱللَّهُ بِهِمْ عَلِيمًا Qur’an An-Nisaa' (4:39)Wamatha AAalayhim law amanoo biAllahi waalyawmi alakhiri waanfaqoo mimma razaqahumu Allahu wakana Allahu bihim AAaleeman
Lit., "what is it that would be upon them". This seems to be a reference to the oft-repeated Qur'anic statement that those who believe in God and live righteously "need have no fear" (la khawf 'alayhim - lit., "no fear [shall be] upon them").
Sustenance: physical, intellectual, spiritual-everything pertaining to life and growth. Our being is from Allah, and we must therefore spend ourselves freely for Allah. How can it be a burden? It is merely a response to the demand of our own healthy nature.