Abu Dawud

955. Commentary:
The best practice is that when the recitation (qira’ah) is performed standing, then the bowing (ruku‘) should also be performed standing; and if the recitation is performed sitting, then the bowing should also be performed sitting... This, and the aforementioned scenario—i.e., if a portion of the rak‘ah is performed standing and a portion sitting—is also permissible.

Hadith Commentary: Benefits and Issues: One mudd (مد) of water amounts to two ratl, which is a quarter of a sa’ (صاع). In this way, the water will be slightly more than one liter, and in three mudd, the water will be less than three and a half liters. From this, it is understood that the Prophet (sallallahu alayhi wa sallam) sometimes managed with three mudd of water instead of three sa’ for ritual bath (ghusl), or in such a case, he may not have performed the ritual bath together (i.e., with someone else). Rather, due to the vessel being small, he would have performed the ritual bath one after the other. According to some, mudd here is in the meaning of sa’.