Step 1: The question asks to identify the source poem of the given line.
Step 2: The line describes a singing bird hidden among mango leaves. "Thou singest" is an archaic way of saying "you sing."
Step 3: This is a direct quote from the poem "Koel" by Puran Singh. The poem opens with this image of the unseen bird whose passionate song is heard from its hiding place in the mango grove.