It is a common scene in the life of Buddha in art, showing Maya asleep on her side having a dream.
[1][2] The story of the dream is recorded in several historic texts like Lalitavistara, the Abiniskramana sutra, the Buddhacarita, the Mahavastu and the Mülasarvastivada Vinaya.
One of the texts says,"A summer festival was organised in the city of Kapilasvastu for seven days before the full moon, Mahämayä had taken part in the festivities On the seventh day she woke-up early, bathed in scented water, and distributed alms, wore splendid clothes and ate pure food followed by performing the vows of the holy day.
"[3] The four guardians of the world lifted her on her couch and carried her to the Himalaya mountains and placed her under a great sal tree .... Then their queens bathed her ... dressed her in heavenly garments, anointed her with perfumes and put garlands of heavenly flowers on her.
The scene shows Queen Maya lying on the couch on her side with female attendants, sleeping on the side and four lokapalas or celestial beings are standing in the corner, an elephant is seen entering the womb of Maya.