A well-known dispute concerns the ʿanʿanah of a non-mudallis: is contemporaneity with a plausible opportunity for meeting enough, or must at least one actual encounter be established?
The introduction to Ṣaḥīḥ Muslim discusses the principle based on contemporaneity and possible meeting. A stricter demand for evidence of encounter is associated with al-Bukhārī, ʿAlī ibn al-Madīnī, and other critics, and Ibn Ḥajar favors that requirement here.
Hypothetical case: Aḥmad and Bakr lived during the same years, but no reliable evidence of meeting is found. Simply being contemporaries is not the same as an express “I heard”; the methodological dispute is about what additional evidence is required.