The word إبن or son. 
Did you ever wonder why sometimes  it’s written as ابن and other times its written as بن?
Well if “son of” comes in a sentence between two proper nouns as a predicate or خبر then the alif will remain. For example:
عمر ابن الخطاب 

“Umar is the son of al-Khattaab” 
Here the word “son” is considered to be a خبر, therefore the if remains.
But if the word “son” is a description or صفة between two proper nouns then the alif is dropped.
For example:
شاهدت الخليفة عمر بن الخطاب 

“I saw the Caliph Umar, the son of al-Khattaab.”
In this instance “son” is no longer the predicate but rather a description to Umar. Therefore, the alif is dropped and it becomes بن. The alif remains*