Far enough to show the land and herd were acquired and built with lawful money. That usually means the original purchase documents, the mortgage and its discharge, and tax returns for the years that produced the capital, together with the recent sale and bank records.
Trace the asset, not just the cheque
Hypothetical example: USCIS asks for evidence of the lawful source of the capital and of its path to the enterprise. Where capital comes from selling an asset, the inquiry extends to how the asset was acquired. For Nora and Tom the recent layer is easy: the sale agreement, closing statement, and the bank statement showing proceeds, followed by the transfer to the project's escrow.
The older layer is the work. They need the land purchase agreements from the years the ranch was assembled, the mortgage documents and discharge, and personal and farm tax returns for as many years as are reasonably obtainable, showing the income that paid down the debt and bought cattle. Where a document from the 1990s cannot be found, a land-registry title history, a lawyer's file copy or a bank confirmation can substitute, with a short signed explanation.
An inheritance that contributed needs the will or estate documents. The aim is a narrative an officer can follow without guessing.