«فَلِاشْتِرَاطِهِ أَنْ يَكُونَ الرَّاوِي قَدْ ثَبَتَ لَهُ لِقَاءٌ مَنْ رَوَى عَنْهُ وَلَوْ مَرَّةً»
It is required that the narrator be known to have met the person from whom he narrates, even once.
The stricter continuity criterion associated with al-Bukhari in discussing muʿanʿan reports stresses demonstrated meeting rather than mere contemporaneity. Evidence of actual contact goes beyond overlap in dates.
Important qualification: proof that two narrators met is not equivalent to explicit proof of hearing every individual report at that meeting. The criterion should not be exaggerated into a supposed demand for the literal “I heard” in each narration.