Here's the exact language of the rule in the 2013 NFL Official Playing Rules that led to the Patriots' 30-27 loss to the Jets on Sunday.
Patriots defensive lineman Chris Jones was called for pushing a teammate during a 56-yard field goal attempt by the Jets in overtime. The 15-yard penalty gave the Jets a first down (the kick was wide), and they then hit a game-winning field goal from 42 yards.
It is Rule 9, Section 1, Article 3 of the official NFL rules.
Defensive Team Formation
(a) When Team A presents a punt, field-goal, or Try Kick formation, a Team B player, who is within one yard of the line of scrimmage, must have his entire body outside the snapper’s shoulder pads at the snap.
(b) When Team A presents a field-goal or Try Kick formation:
(1) No more than six Team B players may be on the line of scrimmage on either side of the snapper at the snap; and
Note: These restrictions do not apply if a team does not present a standard punt, field goal, or Try Kick formation (an equal number of players on the line of scrimmage on either side of the snapper in a tight formation), or if, after the offensive team has assumed a set position, there is a shift, or a player goes in motion.
Penalty: For illegal formation by the defense: Loss of five yards.
(2) Team B players cannot push teammates on the line of scrimmage into the offensive formation.
Penalty: For unnecessary roughness: Loss of 15 yards.