According to Fa Hien "Four places are always determined in advance:
where the Buddhas shall attain Buddhahood;
where they shall begin to preach;
where they shall expound the law and refute heretics;
where they shall descend from the Tushita Heaven after having preached to their mothers;
Other places are chosen according to circumstances."
Lord Buddha Descended from Tushita Heaven
So, the visiting of Lord Buddha to heaven to impart religious discourse and his descendent from the Tushita Heaven was predetermined. As per the legend, it is believed that after preaching in heaven he came down to the ancient city of Sankashya which is located around 50 Kms from Kanauj which was the capital of Harshavardhan's Empire. When he descended on the earth from the heaven, women were encouraged to become the Buddhist monks. It is said that Lord Buddha went to Heaven to teach Abhidharma to his deceased mother Queen Mayadevi who died seven days after his birth and was reborn as male in Tushita. The same thing happened with all the Buddhas who went to teach their mothers and finally descended to Sankashya.
Sankashya - One of the Eight Places of Buddhist Pilgrimage
Sankashya is sixth Buddhist pilgrimage place out of eight which is considered as the most westward and perhaps most obscure of the eight places of pilgrimage. These eight pilgrimage places are Lumbini, Bodh Gaya, Sarnath, Rajgir, Shravasti, Sankashya, Nalanda and Kushinagar. When he retuned back to the Earth a great assembly was called which was composed of kings and the people of the eight kingdoms.
Sankashya – The Ancient Glory
A very little information is available about this place of Sankashya. It is from the accounts of Chinese travelers Huan Shuang and Fa Hien that some information is gathered. It was Fa Hien who found roughly 1,000 monks and nuns; some were Hinayana and some Mahayana living here to pursue their Buddhist studies. Fa Hien especially remarked on the abundant produce of the land and the prosperity and happiness of the people.