Police spokesman Ralf Etzel said Tuesday they are currently looking for two suspects.
Etzel said the 53-year-old rabbi and his sons, both 19, reported that a man approached them and yelled anti-Semitic slurs as they left a central synagogue on Saturday.
A woman in a nearby car joined in, then spat at one of the boys when he approached her.
In the German capital, Berlin, police have been investigating a similar incident in which a prominent rabbi and his young son were spat at and insulted on their way home from synagogue last month.
Germany's domestic intelligence agency in June said the number of anti-Semitic acts of violence rose sharply over the past year alongside a further increase in those identified as far-right extremists.
The BfV agency said in its annual report that the number of anti-Semitic acts of violence rose by 71.4 % in 2018 to 48 from 21 the previous year.
It also said that the number of far-right extremists rose by 100 to 24,100 people last year with more than half of them potentially violent.