Lord Śrī Kṛṣṇa, who manifested His eternal form before the vision of all on the earth, performed His disappearance by removing His form from the sight of those who were unable to see Him [as He is] due to not executing required penance.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
प्रदर्श्यातप्ततपसामवितृप्तदृशां नृणाम् । आदायान्तरधाद्यस्तु स्वबिम्बं लोकलोचनम्
pradarśyātaptatapasāmavitṛptadṛśāṃ nṛṇām | ādāyāntaradhādyastu svabimbaṃ lokalocanam
SB 3.2.11