Question

My question is if the daily prayer is a matter between a person and Allah than why do we address Prophet Muhammad in salutation during the prayer?

Answer

• Refuting misconceptions
The opinion claiming that tawassul [En. supplicating Allah through an intermediary] through the Prophet (peace and blessings be upon him) and asking him to supplicate Allah on one's behalf entails shirk [En. associating partners with Allah] is attributed to the Kharijites. This is an invalid methodology that contravenes the Orthodox Sunni community from among the predecessors and successors.
• Making Tawassul through the Prophet (peace and blessings be upon him) and friends of Allah [Awliya'] is legally permissible

The Companions, their followers and the four followed imams maintain that making tawassul, addressing the Prophet [pbuh] and making a request from him after his death, as well as making tawassul through the friends of Allah and the pious—whether alive or dead—is legally permissible and substantiated by textual evidences, the consensus of scholars and the practice of the predecessors and Successors from among the Muslim community without having anyone deny it.
It was narrated by Abduallah ibn 'Umar (may Allah be pleased with them both) that only the ones who follow the path of the Kharijites contest tawassul; they interpreted the revealed verses to mean Muslims when in fact they refer to those who associate partners with Allah [Reported by al-Bukhari in his Sahih as a suspended report and mentioned by ibn Jureir in Tahdhib al-Athar with an authentic chain of transmission].
This is considered a shortcoming in the methodology of the Kharijites of which Muslims must be aware.

• Evidence
• What every Muslim says in his prayer i.e. his direct address to the Messenger of Allah (peace and blessings be upon him) in Tashahud [testimony of faith]. Speech is only directed to the one who hears it. If it is permissible to address the Prophet during prayers, which is an intimate discourse between Allah and His servant, then a fortiori at other times.
However, the fact that Allah legislated it in prayers proves that the Prophet (pbuh) is the means of tawassul between Him and His servants and that the condition of faith in and acceptance of Allah is only through having faith in the Prophet [pbuh] and accepting him.