Srimad Bhagavatam Canto 10, Chapter 08, Text 50

SB 10.8.50

astv ity uktah sa bhagavan
vraje drono maha-yasah
jajñe nanda iti khyato
yasoda sa dharabhavat
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
When Brahma said, “Yes, let it be so,” the most fortune Drona, who was equal to Bhagavan, appeared in Vrajapura, Vrndavana, as the most famous Nanda Maharaja, and his wife, Dhara, appeared as mother Yasoda.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Because whenever Krsna appears on this earth He superficially needs a father and mother, Drona and Dhara, His eternal father and mother, appeared on earth before Krsna as Nanda Maharaja and Yasoda. In contrast to Sutapa and Prsni, they did not undergo severe penances and austerities to become the father and mother of Krsna. This is the difference between nitya-siddha and sadhana-siddha.
Srimad Bhagavatam Canto 10, Chapter 08, Text 49
Srimad Bhagavatam Canto 10, Chapter 08, Text 51