SB 10.6.30
sri-suka uvaca
iti pranaya-baddhabhir
gopibhih krta-raksanam
payayitva stanam mata
sannyavesayad atmajam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Srila Sukadeva Gosvami continued: All the gopis, headed by mother Yasoda, were bound by maternal affection. After they thus chanted mantras to protect the child, mother Yasoda gave the child the nipple of her breast to suck and then got Him to lie down on His bed.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When a baby drinks milk from the breast of his mother, this is a good sign of health. So the elderly gopis were not satisfied with chanting mantras to give protection to Krsna; they also tested whether their child’s health was in order. When the child sucked the breast, this confirmed that He was healthy, and when the gopis were fully satisfied, they had the child lie down on His bed.