Comparing 2 columns from 2 different spreadsheets

Hello,
Here is what i am trying to do.
Spreadsheet 1 column B has customer numbers
Spreadsheet 2 column A also has customer numbers
Spreadsheet 2 column B has ID numbers

I am trying to write a function that will lookup spreadsheet2, and if it finds a customer number that exists in both sheets, write the ID number from spreadsheet2 in spreadsheet1 column C