Buddha gave first sermon to pancvaggiya- Kaundinya gotra, Mahanaman, Vaspa, asvajit and Bhadrajit at Sarnath. (E. H. Johnson, Buddhacarita, xv-xxviii, p.9 , Asvaghosa's Buddhacarita or Acts of the Buddha, Motilal Banarsidass Publishers Pvt. Ltd. ( 6th reprint Delhi 2015)

Dhammacakkappavattana Sutta.- of the first sermon, preached by the Buddha, to the Pancavaggiyas at Migadaya in Isipatana, on the full moon day of Asajha. (G. P. Malalasekera, Dictionary of Pali Proper Names, Vol- 1, A-DH,Motilal Banarsidass Publishers Pvt. Ltd. ( First Indian reprint Delhi 2007), p.1138)

Buddha subsequently also spent his first rainy season at Sarnath^[8] at the Mulagandhakuti. By then, the Sangha had grown to 60 in number (after Yasa and his friends had become monks), and so Buddha sent them out in all directions to travel alone and teach the Dharma. All 60 monks were Arhats. (https://en.wikipedia.org/wiki/Sarnath)

Buddhism is flourishes in Sarnath because of kings and other wealthy merchants live in Varanasi. Sarnath had become a great center of arts by the third century. In the 7th century, it was noted that 30 monasteries and 3000 monks were founded at Sarnath. (http://www.varanasi.org.in/sarnath)