The death certificate, the will or succession declaration, the probate or settlement act allocating the property, the title record, the sale deed, and the notary's closing and disbursement statements. Bank records then carry the trail forward. Prescribed historical tax records may also be requested alongside these.
Gaps in the chain are the weak point
One weak step is between the estate and the individual heir: the sale is documented, the estate account is documented, but nothing shows the decision that gave you your specific portion. Another is a mortgage discharged at closing with no payoff statement. Missing pieces can produce a request for evidence or a refusal, so close the gaps before filing rather than promising to supply them afterwards.
Grant agreements, vesting statements, brokerage confirmations for each sale, tax returns showing the reported benefit and gains, bank statements for the receiving account and the wire into escrow, reconciled in a single table. Use the dated estate accounts, bank records and notarial disbursements to establish when the funds moved. Bank statements, sale documents, loan instruments and transfer confirmations should permit the capital trail to be followed without unexplained gaps. Bank statements show the proceeds arriving in the parents' account and, later, the gift leaving it and arriving in the investor's account.