Solicitors and legal professionals are subject to money laundering regulations, which basically means that they need to obtain information on the ‘source of funds’ for any transactions they make.
If the money is from equity it’s fairly obvious where that came from, and they’ll usually ask for evidence of where any other funds come from.
If it’s your own savings then they’ll usually ask for proof such as bank statements, if the money was a gift then they need to get evidence of who the gift was from.
They need to make sure funds are not from anyone on a sanction list, and do checks to see if the funds are from anyone who could be considered a ‘Politically exposed person’, which is basically someone who has a higher risk of being a corrupt **** involved with dodgy things.