Salam alaykum.i want to understand the meaning of the statetment 'they have one sent by Allah to question about them' in the following hadith 'avoid sin considered insignificant for they havd one sent by Allah to question about them'

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

What is meant by such sentence, and Allaah Knows best, is that they (the minor sins) are great in the Sight of Allaah The Almighty Who will count them for their doer and may punish him for them. Mulla Al-Qaari said in Al-Mirqaat: "For they have a pursuer from Allaah", means a form of punishment will follow them which cannot be averted for it is a very great pursuer. Thus, one should not neglect it; rather, one should fear it most." [End quote] 

It is obligatory for everyone to be watchful of and fear Allaah The Almighty regarding every major and minor sin. Allaah The Almighty Says (what means): {And the record [of deeds] will be placed [open], and you will see the criminals fearful of that within it, and they will say, "Oh, woe to us! What is this book that leaves nothing small or great except that it has enumerated it?" And they will find what they did present [before them]. And your Lord does injustice to no one.}[Quran 18:49]

Allaah Knows best.