«ضَبْطُ صَدْرٍ: وَهُوَ أَنْ يُثْبِتَ مَا سَمِعَهُ بِحَيْثُ يَتَمَكَّنُ مِنِ اسْتِحْضَارِهِ مَتَى شَاءَ»
Retention in memory means fixing what was heard so firmly that it can be recalled when required.
The requirement is not fulfilled merely by hearing accurately once; the material must remain reliably preserved until transmission. If a narrator's memory deteriorates during a particular period, that chronology may matter for his different transmissions, although its application requires individual biographical analysis.
Example: A student receives an isnad and report from a teacher, preserves them for years, and later reproduces them accurately in a way consistent with reliable evidence. This illustrates dabt al-sadr. Speaking confidently is not enough; the report must actually have been preserved correctly.