Sanjayayana Parva is the fiftieth upa parva included in the fifth maha parva, named as Udyoga parva, in the epic Mahabharata. The original epic was composed by Vyasa in Sanskrit. Vyshampayana had narrated the historical events that had taken place in the life of his fore fathers to Janamejaya on his queries.
Why Dritarashtra had sent Sanjaya to Yudhishtira as an ambassador?
Dritarashtra was afraid of Pandava brothers since he thought that, if a war happens in future, and in that battle, if his son Duryodhana had to face Arjuna or Bhima, then Duryodhana would die. In order to know the real temperament of Pandava brothers, Dritarashtra had sent Sanjaya as a royal ambassador towards Yudhishtira at Upaplava.
Indraprastha was under the control of Yudhishtira thirteen years back, but now under the rule of Duryodhana and it was denied by him to grant it back to Yudhishtira. Hence Dritarashtra had given instructions to Sanjaya to assess the future happenings from the side of Yudhishtira, and try to persuade Pandavas to live peacefully, without entering into his ruling area and not to claim their parental properties.

What was the personal message given to Yudhishtira by Sanjaya?
Sanjaya had visited Upaplava and met with Pandavas. He conveyed all the messages to the concerned persons of the royal family of Kurukshetra, at the palace that was then also called as Nagasahvaya, near the Yamuna River. But Sanjaya had personally opined and told to Yudhishtira that it may be better to live by begging alms, and follow a beggar’s life after their completion of thirteen years of exile in the forests, for the remaining period of their life.