Isaiah 55.3
Septuagint

kai diaqhsomai umin diaqhkhn aiwnion, 
ta osia Dauid ta pista

New Testament - Acts 13.34

oti dwsw umin 
ta osia Dauid ta pista

Septuagint

I will make with you an everlasting covenant, the sure mercies of David

New Testament

I will give you the holy and sure blessings of David
 

Masoretic Text

I will make an everlasting covenant with you, even the sure mercies of David

Comments:  Though the translations differ, the LXX and NT are identical. 

The RSV translates:  “I will make with you an everlasting covenant, my steadfast, sure love for David.”  The NIV:  “I will make an everlasting covenant with you, my faithful love promised to David.”  The NASB:  “And I will make an everlasting covenant with you, According to the faithful mercies shown to David.”  The Hebrew word translated “mercies” in the ASV is chesed, meaning loving kindness.