Tadlis is a subtle defect in the chain where a narrator conceals a flaw, most commonly by reporting from a teacher he met using vague wording like "on the authority of" (ʿan) for a hadith he did not actually hear directly, thereby dropping a weaker intermediary. Such a narrator is a mudallis and the report is mudallas. The scholars of al-jarh wa al-taʿdil scrutinised these narrators closely; a known mudallis's ʿan-report is generally suspended until he explicitly affirms hearing (e.g., "haddathana"). This vigilance flows from the Quranic command to verify what is reported before accepting it.
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.