The hadith is authentic from the Prophet Muhammad (peace be upon him), found in Sahih Muslim 2: 807, and Sahih Bukhari 2: 670. The purpose of it is to emphasize the status of the fasting person, encourage fasting, and indicate that it is beloved to Allah Almighty. It may also imply that people used to avoid speaking to the fasting person due to the change in their breath from fasting, and thus they were advised against that and encouraged to speak.

Refer to: Bada'i' al-Sanai' 2: 106.