Tirmidhi

Explanation:
1:
That is, she had given birth only a few days before this.

2:
This doubt is from the narrator.

Hadith Commentary: Benefits and Issues: It was the practice of the Messenger of Allah (sallallahu alayhi wa sallam) that he would recite lengthy portions of the Qur’an during the night prayer (tahajjud). Until he became advanced in age and his blessed body became heavy, he continued to recite while standing. When the signs of old age became apparent in his disposition and his body became burdensome, then standing for lengthy recitation became difficult. So he (sallallahu alayhi wa sallam) adopted this method: he would perform some units (rak‘ahs) standing and some sitting. And sometimes he did it in such a way that, instead of beginning the recitation while standing, he would start while sitting and then stand up at the end.

Therefore, it is permissible for a person to begin the prayer sitting and then stand up, or to begin the prayer standing and then sit down. Clearly, this need will arise in the situation when one intends to recite at length.