Musannaf Ibn Abi Shayba 22:16

Malik ibn Isma'il narrated to us, he said: Ja'far al-Ahmar narrated to us, from Mutarrif, from Sawada ibn al-Ja'd, from Abu Ja'far, he said: "From a man's understanding (fiqh) is his recognition of the solecism (al-lahn)."