Where does it come from that Khadija (RA) was 40 years old when she married the prophet?

Question

This wiki answer mentions, that Khadija (RA) the business women and first wife of the prophet could be 28 years old, not 40 at the time of her marriage with the prophet

if you google for "The Sons of Khadija" by M. J. Kister (1993), you will find interesting evidence that she was actually only about 28.That makes sense when you consider that she bore Mohammed six children.

I am asking this because even the age of Ayesh (RA) is disputed, but we never dispute Khadija (RA) age. How can we be sure that she was 40 years old at the time of marriage? Is there any reference to this fact?

Answer 1

Actually, there is dispute over how old Khadija was when she married the prophet. It's just not as famous as the one over Aysha's age. Some historians put her as young as 25 while others put her as old as 45. The most wide spread view remains that she was 40 (although the claim that she was 28 is gaining traction).

The main reference for her being 40 is a report attributed to Hakim ibn Hizam (her nephew) where he said:

توفّيت خديجة في شهر رمضان سنة عشر من النبوة ، وهي ابنة خمس وستين

Khadija died in the month of Ramadan in the tenth year of the prophethood, and she was 65

Since we know they were married for 15 years before Mohammad became the prophet, we reach the age 40 when she was married. Many historians however have disputed the narration link to Hakim ibn Hizam and have relied on other reports.

The following article (Arabic) discusses the many theories over Khadija's age.

http://www.aqaed.com/faq/2039/