As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school. But there are five other Astika or orthodox schools of Hindu philosophy: Purva Mimamsa, Samkhya, Yoga, Vaisheshika, and Nyaya. My question is about the Samkhya school, founded by the sage Kapila. Now in Adhyaya 4 of Kapila's Samkhya Sutras, one of the defining texts of the Samkhya school, Kapila discusses various scriptural stories which illustrate different Samkhya teachings. In particular, here's what he says in Adhyaya 4 Sutra 2:
viraktasya heyahānamupādeyopāvānaṃ haṃsacāravat
In the case of the dispassionate, there is the avoidance of what is to be avoided and acceptance of what is fit to be acquired, - as in the case of the swan and milk.
Here is how Vijnanabhikshu interprets this Sutra in his commentary on the Samkhya Sutras:
Perfect development of knowledge takes place in the case only of the dispassionate. In regard to this the author mentions an illustration. Of the dispassionate only, there is avoidance of things to be avoided, such as Prakriti, etc., and the acceptance of that which is to be accepted, i.e. the Self. Just as, out of milk and water, formed into a mixture, by giving up the inessential water, the taking of the essential milk is possible for the swan only, but not for the crow, etc. Such is the meaning.
I'm interested in the part in bold. My question is, what scriptures describe swans separating milk from water? It must be in some scripture or it presumably wouldn't be in Adhyaya 4 of the Samkhya Sutras.
I've come across lots of claims that the term "Paramahansa" used for certain saints comes from this property of swans. For instance here is what Srila Prabhupada says in his commentary on the Srimad Bhagavatam:
A paramahaṁsa accepts only the active principle of everything. Just as a swan accepts only the milk from a mixture of water and milk, a paramahaṁsa accepts only the Supreme Personality of Godhead as his life and soul, neglecting all external, material things.
But does this notion have a scriptural basis?