Treas. Reg. § 1.401(a)-19

(1) General rule. A plan to which this section applies is not a qualified plan (and a trust forming a part of such plan is not a qualified trust) if, under such plan, any part of a participant's accrued benefit derived from employer contributions is forfeitable solely because a benefit derived from the participant's contributions under the plan is voluntarily withdrawn by him after he has become a 50 percent vested participant.

(2) 50 percent vested participant. For purposes of subparagraph (1) of this paragraph, a participant is a 50 percent vested participant when he has a nonforfeitable right (within the meaning of section 411 and the regulations thereunder) to at least 50 percent of his accrued benefit derived from employer contributions. Whether or not a participant is 50 percent vested shall be determined by the ratio of the participant's total nonforfeitable employer-derived accrued benefit under the plan to his total employer-derived accrued benefit under the plan.

(3) Certain forfeitures. Paragraph (b)(1) of this section does not apply in the case of a forfeiture permitted by section 411(a)(3)(D)(iii) and § 1.411(a)-7(d)(3) (relating to forfeitures of certain benefits accrued before September 2, 1974).