﴿ عَلَىٰ قَلْبِكَ لِتَكُونَ مِنَ ٱلْمُنذِرِينَ ﴾

“upon thy heart, [O Muhammad] so that thou mayest be among those who preach”

(Upon your heart) according to your memorisation; it is also said that this means: when it was recited to you, (that thou mayst be (one) of the warners) by means of the Qur'an,