On the fifteenth of the holy month of Ramadhan, in the third year after Hijrah (migration), the Prophetic house announced the birth of the first grandson of the Prophet [s]. This good news was given to the Chosen Prophet [s]. A look of joy covered his noble face and happiness flooded his heart. He hastened to the house of his daughter, the chaste Zahra, to communicate his congratulations and show his delight.
The blessed newborn baby was taken to him, in the arms of Um-Salamah, or in another version, Asma', daughter of Umays. The Prophet [s] received him with the whole of his pure existence. He carried him in his arms. He kissed him and pressed him to his chest. Then he said the call to prayer (adhan) in his right ear and recited the call to begin the prayer (iqamah) in his left ear. The first voice that embraced the newborn's hearing and entity was, thus, the voice of right.
The Holy Prophet [s] turned to Imam Ali [a] and asked him:
"What name did you give my son?"
"I wouldn't precede you in doing it," Imam Ali [a] replied.
"Nor would I precede my Lord," averred the Prophet [s]. 
No sooner did this short dialogue take place between the Messenger [s] and his trustee, about the name of the new baby, than the Divine, Holy revelation came to the Messenger of Allah [s] telling him that Allah, the Glorified, had named the baby, Hasan. 
That was the first phase of the Islamic rituals in honouring Imam Hasan [a], the noble baby.
 . Ibid., p.120.
 Tawfeeq Abu-Alam, Ahlul Bayt/Imam Hasan, 1st ed., 1970, p.264. Al-Majalis al-Saniyyah (Bright Assemblies), vol.2, Sayyid Muhsin al-Ameen al-Amili, Life of Imam Hasan [a].