Baby refusing breastmilk

Did not have any breastmilk for first 4 days and baby was at risk of jaudice coz no intake. So we started giving formula milk. Now that BM has kicked in, we wanted feed baby BM together with FM (coz BM amount still very low) but baby is rejecting the BM (cries when we feed BM), so we mixed BM and FM. After an hour of feeding, baby kept crying... how to get baby to go back to BM and away from FM? Would like to exclusively feed BM after amount is huge enough to support full BF.