1040 Line 25c has an amount but turbotax will not tell me the source; how do I figure out where that amount came from?

Line 25 C includes taxes withheld from other sources of income for example Forms W-2G, 8959 Additional Medicare Tax, 1042-S, 8805, 8288-A or Schedule K-1, Partner’s Share of Income, Deductions, Credits, 

Refer to IRS instructions for 1040 line 25 C for more information.