One day mother Yaśodā, having taken Kṛṣṇa up and placed Him on her lap, was feeding Him milk from her breast with maternal affection. The milk was flowing from her breast, and the child was drinking it.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
एकदार्भकमादाय स्वाङ्कमारोप्य भामिनी । प्रस्नुतं पाययामास स्तनं स्नेहपरिप्लुता
ekadārbhakamādāya svāṅkamāropya bhāminī | prasnutaṃ pāyayāmāsa stanaṃ snehapariplutā
SB 10.7.34