A “Source of Funds” confirmation is required only if you live in, or transfer your blocked amount in full or in part from a country that is considered "high-risk" per the EU money laundering regulations or the regulations of the German Financial Supervisory Authority (BaFin).
A list of all such countries can be found here.
If applicable to you, the Fintiba app will seamlessly guide you through the Source of Funds verification process.