Isaiah 61:1
1 The Spirit of the Sovereign Lord is upon me, because the Lord has chosen me. He has commissioned me to encourage the poor, to help the brokenhearted, to decree the release of captives and the freeing of prisoners, 2 to announce the year when the Lord will show his favor, the day when our God will seek vengeance, to console all who mourn, 3 to strengthen those who mourn in Zion by giving them a turban, instead of ashes, oil symbolizing joy, instead of mourning, a garment symbolizing praise, instead of discouragement. They will be called oaks of righteousness, trees planted by the Lord to reveal his splendor.
Matthew 5:3
1 When he saw the crowds, he went up the mountain. After he sat down his disciples came to him. 2 Then he began to teach them by saying: 3 “Blessed are the poor in spirit, for the kingdom of heaven belongs to them. 4 Blessed are those who mourn, for they will be comforted. 5 Blessed are the meek, for they will inherit the earth. 6 Blessed are those who hunger and thirst for righteousness, for they will be satisfied. 7 Blessed are the merciful, for they will be shown mercy.
Notes and References
"... The expression “blessed are” appears sixty-eight times in the LXX, usually for the Hebrew ashrei (“happy are ...”; see, e.g., Psalm 84.4, “Happy are those who live in your house, ever singing your praise. Selah”) known from the Jewish liturgy, where it introduces Psalm 145. In terms of content as well, the Beatitudes draw upon scriptural precedent. For example, “Blessed are the poor in spirit” and “Blessed are those who mourn” may be dependent on Isaiah 61:1–3, which also speaks of the poor and those who mourn ..."
Levine, Amy-Jill & Brettler, Marc Zvi The Jewish Annotated New Testament (p. 10) Oxford University Press, 2011