Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 43
एवमेतेषु भेदेषु चिरं ध्यात्वा स आत्मभू: ।सत्या: के कतरे नेति ज्ञातुं नेष्टे कथञ्चन ॥ ४३ ॥
evam eteṣu bhedeṣuciraṁ dhyātvā sa ātma-bhūḥsatyāḥ ke katare netijñātuṁ neṣṭe kathañcana
Thus Lord Brahmā, thinking and thinking for a long time, tried to distinguish between those two sets of boys, who were each separately existing. He tried to understand who was real and who was not real, but he couldn’t understand at all.
Brahmā was puzzled. “The original boys and calves are still sleeping as I have kept them,” he thought, “but another set is here playing with Kṛṣṇa. How has this happened?” Brahmā could not grasp what was happening. Which boys were real, and which were not real? Brahmā was unable to come to any definite conclusion. He pondered the matter for a long while. “How can there be two sets of calves and boys at the same time? Have the boys and calves here been created by Kṛṣṇa, or has Kṛṣṇa created the ones lying asleep? Or are both merely creations of Kṛṣṇa?” Brahmā thought about the subject in many different ways. “After I go to the cave and see that the boys and calves are still there, does Kṛṣṇa go take them away and put them here so that I come here and see them, and does Kṛṣṇa then take them from here and put them there?” Brahmā could not figure out how there could be two sets of calves and cowherd boys exactly alike. Although thinking and thinking, he could not understand at all.