What is the ruling if someone realizes they omitted a prostration during the Tashahhud?

Chapter on the Two Prostrations of Forgetfulness

Al-Mughni

Book of Prayer

Book 3 · Issue 9 · Bab 7

Open in Qurani

Primary text

If someone realizes during the Tashahhud that they omitted a prostration from a rak'ah, they must perform that rak'ah with its two prostrations and then perform Sujud as-Sahw.