We discovered a strongly lensed ( \mu \gtrsim 40 ) Ly \alpha emission at z=6.629 ( S/N \simeq 18 ) in the MUSE Deep Lensed Field ( MDLF ) targeting the Hubble Frontier Field galaxy cluster MACS J0416 .
Dedicated lensing simulations imply that the Ly \alpha emitting region necessarily crosses the caustic .
The arc-like shape of the Ly \alpha extends 3 ^ { \prime \prime } on the observed plane and is the result of two merged multiple images , each one with a de-lensed Ly \alpha luminosity L \lesssim 2.8 \times 10 ^ { 40 } erg s ^ { -1 } arising from a confined region ( \lesssim 150 pc effective radius ) .
A spatially unresolved HST counterpart is barely detected at S/N \simeq 2 after stacking the near-infrared bands , corresponding to an observed ( intrinsic ) magnitude m _ { 1500 } \gtrsim 30.8 ( \gtrsim 35.0 ) .
The inferred rest-frame Ly \alpha equivalent width is EW _ { 0 } > 1120Å if the IGM transmission is T _ { IGM } < 0.5 .
The low luminosities and the extremely large Ly \alpha EW _ { 0 } match the case of a Population III star complex made of several dozens stars ( \sim 10 ^ { 4 } M _ { \odot } ) which irradiate a H ii region crossing the caustic .
While the Ly \alpha and stellar continuum are among the faintest ever observed at this redshift , the continuum and the Ly \alpha emissions could be affected by differential magnification , possibly biasing the EW _ { 0 } estimate .
The aforementioned tentative HST detection tend to favour a large EW _ { 0 } , making such a faint Pop III candidate a key target for the James Webb Space Telescope and Extremely Large Telescopes .