My parents taught me the kalimah shahadah as follows: la ilaha illAllah Muhammad ur rasulullah. Why do I have to add that Prophet Mohammed is the Messenger of Allah while there were so many prophets and all of them taught the same religion? Would it matter if I just said la ilaha illallah?
Why do we mention the name of the Prophet Muhammad (sws) and the Prophet Ibrahim (sws) in the tashahhud (sitting position in the prayer)? Also please tell me who authored the supplication to be made in this position?
We add the second part of shahadah specifically mentioning the name of the Prophet Muhammad (sws) because we happen to live in the time when his message and the book he brought is the source of guidance for us. We are addressees of the Prophet Muhammad (sws). It is our position as his addressee that makes it necessary to acknowledge his status and the Messenger of Allah. When we declare belief in his prophethood we acknowledge the belief in all the messengers.
The Prophet (sws) taught the supplications to be said in the tashahhud. However they are not the only supplications. We have a choice here. We do not think there is a problem in naming the Messenger in this context. We address him metaphorically. We do not mean to say that he is present among us.
As for the name of the Prophet Abraham (sws) that is only for the sake of reference. The Qur'an tells us that he was blessed greatly by God. You may know that after severe tests and trials he was blessed by confining the institution of Prophethood in his progeny. This is a great blessing of God. All the three major religions in the world hold him in great esteem. Many Messengers came from among his children including the Prophet Muhammad (sws). We seek equally great blessings of God for the Prophet Muhammad sws.