I heard from some preachers that the age of Khadija when she got married to
our Prophet (saw) mentioned differently in the classical books of seerah.
Also it is said that only one of those mentions that she was 40, but other
give different ages even 28 or 29. Could you please give me any name of
those books? I heard that Ibn Ishaq in his seerah mentions this, but the
translation I found online says Muhammad married Khadija, who was some
fifteen years older than he.
The most authentic narrations state that the Prophet alayhi as salam
was 25 and Khadija was 40 when they were married. There are some weak narrations that say she was younger.
As for Ibn Ishaaq’ seerah, it is known to have many weak narrations. Ibn Hisham pointed out some of these weak narrations.
Perhaps you can refer to Ibn Hisham’s Seerah for more details on this issue.
And Allah knows best.