The Quran

Preface

أَفَمَن كانَ عَلىٰ بَيِّنَةٍ مِن رَبِّهِ وَيَتلوهُ شاهِدٌ مِنهُ وَمِن قَبلِهِ كِتابُ موسىٰ إِمامًا وَرَحمَةً ۚ أُولٰئِكَ يُؤمِنونَ بِهِ ۚ وَمَن يَكفُر بِهِ مِنَ الأَحزابِ فَالنّارُ مَوعِدُهُ ۚ فَلا تَكُ في مِريَةٍ مِنهُ ۚ إِنَّهُ الحَقُّ مِن رَبِّكَ وَلٰكِنَّ أَكثَرَ النّاسِ لا يُؤمِنونَ

Yusuf Ali

an they be (like) those who accept a Clear (Sign) from their Lord, and whom a witness from Himself¹ doth teach, as did the Book of Moses before it,- a guide² and a mercy? They believe therein; but those of the Sects that reject it,- the Fire will be their promised meeting-place. Be not then in doubt thereon: for it is the truth from thy Lord: yet many among men do not believe!

“A witness from Himself"', i.e., the Book which was given to al Muṣṭafā, the Holy Qur-ān, which is compared to the original Revelation given to Moses. We make no difference between one true and genuine Message and another, nor between one prophet and another?for they all come from the One True God.
“Guide”: the Arabic word here is Imām, a leader, a guide, one that directs to the true Path. Such a direction is an instance of the Mercy and Goodness of God to man. The Qur-ān and the Prophet Muḥammad are also called, each, a Guide and a Mercy, and so are these epithets applicable to previous Books and Prophets.