When is a Muslim obliged to adopt the saying of a Companion or an Imam, and how does one reconcile this with the principle of following the Quran and Sunnah?

One must follow the Qur’an and Sunnah. Taking knowledge from the Companions and scholars known for their knowledge and virtue is part of this adherence, in accordance with His, the Almighty’s, saying: (So ask the people of the message if you do not know). The saying of a scholar is either in agreement with the Sunnah, in which case it is accepted, or it contradicts it, in which case it is rejected. The rule is that everyone's words can be taken or left, except for the Messenger, peace and blessings be upon him.

The saying of a Companion is considered a proof by many scholars unless it contradicts a text or another Companion's saying, because they were most knowledgeable of the revelation and understood the Qur’an and Sunnah best. As for scholars other than the Companions, they are the most knowledgeable people of the Qur’an and Sunnah and most capable of deriving rulings.

If the scholars agree on a religious ruling, it is a legal proof. If they differ, the reference point is the Qur’an and Sunnah; thus, what conforms to them is followed, and what contradicts them is rejected, while appreciating and respecting the scholars.