Season 5 Episode 4; 

Burrell talking to Nerese about Daniels when Burrell realises his job is under threat by getting replaced by Daniels:

> Burrell: He's not the spick and span boy they think he is. He came up in the Eastern District, part of a bad drug unit skimming seized drug money. I actually had the Feds...  
Nerese: [interrupts] This isn't about Daniels...  
...  
  
Edit: Just wanted to add that in one of the earlier seasons and episodes, Carver and Herc were 'caught' taking drug money for themselves when the recording off the wire didn't match the amount seized (the bin bag of cash ripped in the trunk of the car leaving a wad of cash behind). Daniels picked on the discrepancy and came down hard on Carver and Herc. With the Burrell information above in mind, it sounds like this is the kind of thing Daniels was involved in or associated with and wanted to prevent Carver and Herc from dirtying themselves and ruining their careers and maybe further dirtying Daniels.