I found it in one choir in Mussorgsky's Хованщина. The song goes like this:
Плывёт, плывёт лебёдушка, ладу, ладу,
плывёт навстречу лебедю, ладу, ладу.
etc.; the „ладу, ладу“ is regularly repeated at the end of every line. What does it mean? (And does it mean anything at all, or is it only an interjection like "fa la la" in English?)
E: Thank you both. Since they both are very good, I just accepted one randomly.