What is the meaning of "utterance of the heart," and why is utterance attributed to the heart despite its function being affirmation and acknowledgment, while the tongue's function is articulation?

The phrase "utterance of the heart" was used by the predecessors of the Ummah in defining faith, by which they meant belief and affirmation. Evidence for this includes:

1. The Almighty's saying: "Indeed, the soul is a persistent inciter of evil."

2. The hadith of Abu Hurairah that the Prophet, peace and blessings be upon him, said: "..." (The complete hadith is not mentioned in the original text).

3. The saying of Abu Ad-Darda: "Let one of you beware lest the hearts of the believers curse him while he is unaware."

4. The saying of Al-Hasan Al-Basri: "The people of knowledge have continuously returned to remembrance through reflection, and to reflection through remembrance, and they converse with the hearts until, when they speak, they possess hearing and sight. Thus, they speak with knowledge and inherit wisdom."

5. The saying of Al-Junayd: "Tawhid is the utterance of the heart."