Tottoko Hamtarou OVA 1: Hamtaro no Otanjoubi ~Mama wo Tazunete Sanzen Techi Techi~

Laura tells Hamtaro where he came from yet Laura thinks it is not true, just in time for his birthday, Hamtaro sets out with Boss and Oxnard to visit his mother to see if what Laura said is true, while the other Ham-Hams set up a surprise birthday party for him.
(Source: Wikipedia)