Srimad Bhagavatam Canto 04, Chapter 17, Text 03

SB 4.17.3

vidura uvaca
kasmad dadhara go-rupam
dharitri bahu-rupini
yam dudoha prthus tatra
ko vatso dohanam ca kim
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
Vidura inquired from the great sage Maitreya: My dear brahmana, since mother earth can appear in different shapes, why did she take the shape of a cow? And when King Prthu milked her, who became the calf, and what was the milking pot?
Srimad Bhagavatam Canto 04, Chapter 17, Text 02
Srimad Bhagavatam Canto 04, Chapter 17, Text 04