Gita Mahatmya
Mahābhārata, Bhīṣma-parva 43.1
These verses glorify the Bhagavad-gītā. They come from other scriptures and are not verses of the Bhagavad-gītā itself. Each one carries its own source.
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Gītā-māhātmya
गीता सुगीता कर्तव्या किमन्यैः शास्त्रसंग्रहैः ।
या स्वयं पद्मनाभस्य मुखपद्माद् विनिःसृता ॥gītā sugītā kartavyā kimanyaiḥ śāstrasaṁgrahaiḥ
yā svayaṁ padmanābhasya mukhapadmād viniḥsṛtāOne should sing and study the Gītā well. What is the need for many other scriptures? The Gītā came from the lotus mouth of Lord Padmanābha (Viṣṇu) Himself.
Explanation
The same verse, with the word śāstra-vistaraiḥ, is Gītā-māhātmya 4 in Bhagavad-gītā As It Is.
Source: Mahābhārata, Bhīṣma-parva 43.1