im not sure if i agree with the above statement. Was Abraham given a holy book? if he did not , then im sure the way he performed salat was something that he did it according to how Allah told him. Hence, again according to you.. the man-made salaah, because people must have recorded how he was performing his salaah.
We have been given a method of Salaah which no other prophet before MuhammadPBUH had been given. Had it been salaah the same way as now, there was no need of MuhammadPBUH to come. Jesus was from a family of Abraham as well, but was not given the Muhammadiyya laws. For sure, the message was the same, but that message was for people of that age and period. Now, its only Islam, and to understand Islam, one has to rely on some ahadith clarifying different issues of Quran.