FMFCTR doesn't keep this linkage either. The fund source you see there is merely a default one. However, the real link is established only in budget structure, if you use one. If you don't use budget structure, then the link is defined post factum. The table for budget addresses is not transparent and you won't be able to see the links just by browsing the records.
P.S. Table for budget is FMBDT (totals) and FMAVCT (for availability control)