Imam Baihaqi (R) mentions a narration of Hadhrat Abbas Ibn Abdul Muttalib (RA), the paternal uncle of Nabi (S), regarding the motivating factors behind him accepting Islam.
Abbas (RA) mentions he once said to Nabi (S), “Oh Rasulullah (S)! I was drawn towards accepting your Deen due to a sign (which I had witnessed, proving the truthfulness) of your Prophethood. I had witnessed you in your cradle, conversing with the moon, indicating towards it with your finger. In which ever direction you would point to, the moon would move accordingly.” Nabi (S) mentioned, “I would speak to the moon, and the moon would speak to me, thus diverting my attention away from crying. I would also hear the sound of the moon setting, whilst prostrating before the throne of Allah Subhanahu Wa Ta’ala.”
(Dala’ilun-Nabuwwah by Imam Baihaqi – v. 2, pg. 41)
Imam Suyuti (R) quotes Ibn Sab’a (R) as mentioning that the cradle of Nabi (S) would gently rock by the rocking of the Angels. He further mentions that the first words spoken by Nabi (S) were:
“Allahu Akbaru Kabeera Wal Hamdulillahi Kathiraa.”
‘Allah is The Greatest and all Praise is due to Allah in abundance.’
(Al-Khasa’isul-Kubra by Imam Suyuti – v.1, pg. 53)