Today 15-July-2016, As per Hindu calendar, the day is being celebrated as Dev-Shayani Ekadashi or देवशयनी एकादशी.
Note that this is different from the usual Ekadashi which is discussed in this post on Hinduism.SE
As per Wikipedia, On this day images of Vishnu and Lakshmi are worshipped, the entire night is spent singing prayers.
It is believed that Vishnu falls asleep in Ksheersagar - cosmic ocean of milk - on Shesha nāga, the cosmic serpent. That is why the day is also called Dev-Shayani Ekadashi.
Vishnu finally awakens from his slumber four months later on Prabodhini Ekadashi. And this 4-month period is also called as Chaturmas (meaning "four months")
My Question is : Are there any Hindu scriptures which mentions about this day when Vishnu falls asleep in Ksheersagar for the duration of 4-Months?