On deviation (ziygh): it means turning away from the truth and the corruption of the heart.
Overall meaning: Imam Ahmad denounces anyone who, after recognizing an authentic hadith from the Messenger of Allah, then abandons it and follows the opinion of Sufyan or any other jurist in contradiction to that hadith, offering baseless excuses to justify his action. The obligation that is both compulsory and unequivocal for every believer—once the Book of Allah and the Sunnah of His Messenger reach him, and he understands their purport in any matter—is to act upon them, even if others dissent. Our Lord has commanded us to do so, and so has our Prophet. Imam Ahmad therefore fears that a person who is certain about a Sunnah of the Messenger of Allah but thereafter opposes any part of it may have his heart led astray and be ruined in this world and the Hereafter. He substantiates this with the cited verse, and many similar passages occur in the Quran, such as: “So when they deviated, Allah caused their hearts to deviate” (al-Saff 61:5).
Relevance to the chapter: It warns against following scholars without evidence and against abandoning the Quran and Sunnah, for such conduct constitutes associating others with Allah in the matter of obedience.
Lessons derived from this report:
- Imitation (taqlid) is forbidden for anyone who knows the proof and how to apply it.
- Imitation is permissible for one who does not know the proof, provided he follows a scholar whose knowledge and piety he trusts.