When employers need documents ?

[UK007]

Hi, my visa is running out end of this month. I was under the impression that as long as I provide my employer a letter from UKVI about my visa application, before the expiry date that should be ok. But today I was told by my employer that I have to give something 3 weeks before my visa expiry ! Is it some kind of new rules or something ?

If I am right, can you provide me any link where it says that, so that I can show that to my employer & ask not to give me headache now...