Psalm 95.7-8
Septuagint

oti autoV estin o qeoV hmwn, kai hmeiV laoV nomhV autou, kai probata ceiroV autou: 
shmeron ean thV fwnhV autou akoushte, 
mh sklhrunhte taV kardiaV umwn, 
wV en tw parapikrasmw,
kata thn hmeran tou pikrasmou en th erhmw 
 

New Testament - Hebrews 3.15

en tw legesqai: 

shmeron ean thV fwnhV autou akoushte, 
mh sklhrunhte taV kardiaV umwn 
wV en tw parapikrasmw

Hebrews 4.7

palin tina orizei hmeran, shmeron, en Dauid legwn meta tosouton cronon, kaqwV proeirhtai: 
shmeron ean thV fwnhV autou akoushte, 
mh sklhrunhte taV kardiaV umwn

Septuagint

For he is our God; and we are the people of his pasture, and the sheep of his hand.  Today, if ye shall hear his voice, harden not your hearts, as in the provocation, according to the day of irritation in the wilderness

New Testament

3.15:  while it is said, Today if ye shall hear his voice, Harden not your hearts, as in the provocation

4.7:  he again defineth a certain day, Today, saying in David so long a time afterward (even as hath been said before), Today if ye shall hear his voice, Harden not your hearts

Masoretic Text

For he is our God, and we are the people of his pasture, and the sheep of his hand.  Today, oh that ye would hear his voice!  Harden not your heart, as at Meribah, As in the day of Massah in the wilderness

Comments:  The MT includes the place name from Exodus 17.7 where “contention = Meribah” occurred.  The LXX and the NT employ “provocation” instead.

Hebrews 4.7, however, does not quote that portion of the passage.  However, there is a real difference between the LXX and the NT’s “if ye shall hear his voice” and “oh that ye would hear his voice” from the Hebrew.  The latter construction would come into Greek through a verb in the optative mood.  But the verb akoushte is subjunctive.

The RSV gives this phrase as “O that today you would hearken to his voice!”  Other translations make the Hebrew read very much like the Greek.  For instance, the NIV has “if you hear his voice.”