Prabhosa or Prabhagiri is a hill located quiet close by to Kaushambi which is one of the holy spots for the Buddhists. In earlier times, this hill was known by the name of Mankula. Lord Buddha spent his sixth Rains retreat here at this hill. What is noticeable is that there are no discourses delivered here by the Buddha. This might sound very strange to some, but a visit to Prabhosa is sufficient to dispel all doubts. Prabhosa is an extremely tranquil place and is also very lonely.

When Hieun Tsang came here during the course of his travel, he found a stupa that was constructed by Emperor Ashoka. Today, there is no stupa, however, what one can find a number of caves and rock shelters. Most of these caves and rock shelters are found in the north side of the hill which is both steeper as well as rockier.

Prominent amongst all these caves is the Sita’s Window. This cave is not just largest of all the caves but is also believed to be the one where the Buddha resided during his stay.

Source: http://www.buddhist-tourism.com/countries/india/buddhist-sites/prabhosa-uttar-pradesh.html