This is authentic.
It is noteworthy to say that the translation is wrong and it is not “after
the obligatory prayers” as this is an innovation.
The correct translation is: “at the end of obligatory prayers”! This means before Salam.

Sheikh, is this authentic? Abu Umama al-Bahili (Allah be pleased with him) narrates: The Messenger of Allah (Allah bless him & give him peace) was asked as to which supplication (dua) was most quickly accepted? He replied: “In the middle of the night and after the obligatory (fard) prayers.”