At the age of 40, while engaged in a meditative retreat, Muhammad,peace be upon him, received his first revelation from God through the Angel Gabriel. This revelation, which continued for 23 years, is known as the Quran. As soon as he began to recite the words he heard from Gabriel, and to preach the truth which God had revealed to him, he and his small group of followers suffered bitter persecution which grew so fierce that in the year 622 God gave them the command to emigrate. This event, the Hijra, 'migration', in which they left Makkah for the city of Madinah some 260 miles to the north, marks the beginning of the Muslim calendar.