SB 7.10.18

śrī-bhagavān uvāca
triḥ-saptabhiḥ pitā pūtaḥ
 pitṛbhiḥ saha te ’nagha
yat sādho ’sya kule jāto
 bhavān vai kula-pāvanaḥ
Synonyms: 
śrī-bhagavān uvāca — the Supreme Personality of Godhead said; triḥ-saptabhiḥ — seven multiplied by three (that is to say, twenty-one); pitā — father; pūtaḥ — purified; pitṛbhiḥ — with your forefathers; saha — all together; te — your; anagha — O most sinless personality (Prahlāda Mahārāja); yat — because; sādho — O great saintly person; asya — of this person; kule — in the dynasty; jātaḥ — took birth; bhavān — you; vai — indeed; kula-pāvanaḥ — the purifier of the whole dynasty.
Translation: 
The Supreme Personality of Godhead said: My dear Prahlāda, O most pure, O great saintly person, your father has been purified, along with twenty-one forefathers in your family. Because you were born in this family, the entire dynasty has been purified.
Purport: 

The word triḥ-saptabhiḥ means seven multiplied by three. In one’s family one can count back four or five generations — to one’s great-grandfather or even one’s great-grandfather’s father — but since the Lord mentions twenty-one forefathers, this indicates that the benediction expands to other families also. Before the present family in which one has taken birth, one must have been born in other families. Thus when a Vaiṣṇava takes birth in a family, by the grace of the Lord he purifies not only that family but also the families of his previous births.