It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Jibril came to me and placed his two hands, one on my chest and the other between my shoulders, until I felt the coolness of the hand that was on my chest between my shoulders, and of the one that was between my shoulders in my chest. Then he said: O Muhammad! Declare the greatness (say Allahu Akbar), declare the oneness with certainty (say La ilaha illallah), and say: Glory be to the Lord of the first and the last."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7865