My DM says that the wolf's bite attack causes a Str save or be knocked prone regardless of whether it hits or not.
The attack is:
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (2d4 + 2) piercing damage. If the target is a creature, it must succeed on a DC 11 Strength saving throw or be knocked prone.
I thought it was implicit in the reading that the attack must first hit?
The knockdown only happens on a successful hit.
Everything between the attack name and “Hit:” is general to the attack; everything after “Hit:” happens only if the attack hits.
This is detailed explicitly in the Introduction of the Monster Manual, where it explains how to read monsters' stats, so you don't have to rely on implicit reading at all. On page 11 under Actions, Melee and Ranged Attacks (headings are on previous page) it says:
Hit. Any damage dealt or other effects that occur as a result of an attack hitting a target are described after the "Hit" notation.
In the SRD v5.1, p. 251:
Hit. Any damage dealt or other effects that occur as a result of an attack hitting a target are described after the “Hit” notation. You have the option of taking average damage or rolling the damage; for this reason, both the average damage and the die expression are presented.
The sentence that starts "If the target is a creature..." comes after "Hit:", therefore it is part of the effect that comes from the bite attack hitting.
