Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.156A.

This refers to a quick sitting that one makes after the second prostration of the first and third rak’ah. The scholars differ over this regulation due to the differing hadith. Says Ibn al-Qayyim, “The jurists differ over this act. Is it a sunnah of the prayer that one should perform, or is it only done due to some necessity? There are two statements on this question and two narrations from Ahmad. Said al-Khallal, ‘Ahmad referred to the hadith of Malik ibn al-Huwairith regarding the intermediate position of rest (between the two prostrations). He said, ‘Yusuf ibn Musa informed me that Abu Umamah was asked about standing up (in the prayer) and he said, ‘It should be done on the tops of the feet according to the hadith of Rafa’. In the hadith of Ibn ‘Ajlan there is no proof that he would stand on the tips of his feet. Many of the companions and others who described the prayers of the Prophet did not mention this sitting, except in what is related by Abu Humaid and Malik ibn al-Huwairith. If it was part of his guidance, he would always do it, and those who described his prayers would have mentioned it.

The fact that he may have done so does not necessarily make it one of the sunnahs of the prayer, unless he did it as a regular practice for the people to follow. Otherwise, he may have done it out of some need to do so, and this would not prove that it is a sunnah of the prayer.”