What is the ruling if a worshipper verbally replies to someone's Salaam during prayer?

Invalidations of Prayer (مفسدات الصلاة) Part 1: Speech and Sounds

• The prayer remains valid but is disliked.
• The prayer is immediately invalidated.
• Worshipper must perform Sujud al-Sahw.
• The prayer is only broken if the reply is loud.