Shaikh Taqi ud din al-Hilali said,
“The Unity of following the Messenger of Allah, Tawhid al-Ittiba and this is included in the meaning of ‘I testify that Muhammad (peace and blessings of Allah be upon him) is the Messenger of Allah and that means ‘none has the right to be followed after Allah’s Book (orders) in the Quran but the Messenger of Allah.”
Shaikh Taqi ud din al-Hilali went on to say,
“A confession with your heart that you have to say, ‘O Allah! I testify that Muhammad (SalAllahu Alayhi Wasallam) is Your Messenger.’ That means that none has the right to be followed after Allah but the Prophet Muhammad (p.b.u.h), as he is the last of His Messengers. As for others than Muhammad (SalAllahu Alayhi Wasallam), their statements are to be taken or rejected as to whether these are in accordance with Allah’s Book (i.e. the Quran) or with the traditions of the Prophet (SalAllahu Alayhi Wasallam)(i.e. the Sunnah) or not.
He further added saying, “So whoever has confessed this (the Shahaada with His Lord), he shall not commit sins like robbing, killing, stealing, illegal sexual intercourse, eating pig’s meat, drinking alcoholic beverages, taking undue advantage of orphan’s property, cheating in trade, bribery and earning money through illegal means, etc. or otherwise the limbs and the organs of his body will testify against him that he was a liar in his words which he pledged to Allah.” [See Tafsir al Quran al-Karim by Shaikh al Hilali, in English, German and Arabic pg.14-17]