That is, have peace of mind, O Muhammad! Have peace of mind, O Muslim! Have complete confidence in this Book which brings to you the path to felicity in this life and the Hereafter! Because Allah (swt) says:
﴾ لَّا يَأْتِيهِ الْبَاطِلُ مِن بَيْنِ يَدَيْهِ وَلَا مِنْ خَلْفِهِ ۖ تَنزِيلٌ مِّنْ حَكِيمٍ حَمِيدٍ﴿
(Falsehood cannot come to it from before it or behind it. (It is) sent down by the All-Wise, Worthy of all praise. ) (Fussilat: 42)
It is a book which has, amongst its characteristics, that it is recorded in honored sheets. This is because, it was sent from the sky and was transcribed from the (The Preserved Tablet).