Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The mouth in a dream is the key to a man's affairs, their conclusion, the source of his sustenance, and his secret. Its large size indicates high status, and its small size indicates lowliness. Something good coming out of it indicates good speech and useful knowledge. Something bad coming out of it indicates ugly speech and backbiting.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The mouth indicates sustenance, knowledge, and guidance. Whoever sees his mouth is sealed will die in a state of disbelief. Whoever sees himself eating from another's mouth will receive goodness from him.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The mouth is interpreted as the house or the treasury. What comes out of it indicates what is in a person's inner self. A beautiful mouth indicates good praise.