«وَأَمَّا مُدْرَجُ الْمَتْنِ فَهُوَ أَنْ يَقَعَ فِي الْمَتْنِ كَلَامٌ لَيْسَ مِنْهُ»
Matn interpolation occurs when words appear within a hadith text even though they were not originally part of that text.
The insertion may occur at the beginning, middle, or end, with final additions especially common. The inserted material may be a narrator's gloss, an independent Companion statement, or a fragment of another conversation.
Example: A Companion comments, “This practice is beneficial,” after a prophetic report. If a later transmission joins the comment to the report without separation, that comment is mudraj. Proper criticism preserves both the original raised wording and the independent attribution of the addition.