<bdi><a href="//commons.wikimedia.org/wiki/Category:Carl_Ludwig_Hummel_de_Bourdon" title="Category:Carl Ludwig Hummel de Bourdon">Carl Ludwig Hummel de Bourdon</a>
</bdi>
<a href="https://en.wikipedia.org/wiki/Christie%27s" class="extiw" title="en:Christie's">Christie's</a>, LotFinder:  entry  <a rel="nofollow" class="external text" href="https://www.christies.com/LotFinder/lot_details.aspx?intObjectID=5743560">5743560</a> (sale 1162, lot 193, London, 26 - 27 November 2013, page no more available)