Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.123A.

We have already stated the Qur’anic verse dealing with this obligatory act. The Prophet explained it in a hadith by saying, “Then prostrate until you attain calmness in your prostration, then rise (and sit) until you attain calmness in your sitting, and then prostrate until you gain calmness in your prostration. The first prostration, sitting afterwards, the second prostration and calmness during all of these acts are obligatory in every rak’ah of every obligatory or superogatory prayer.