This outfit features a very pale pastel blue top which has full length sleeves, just above knee length, straight bottom seam, seamed facing and covered front fastenings (probably a zip). This shirt is quite tight fitting and I would have chosen to wear a larger size so that it was less form fitting especially considering it features visible seams over the bust. This shirt is worn over a long sleeved khaki tee but should not be necessary with the shirt options below as they are opaque and long sleeved.
The color of the shirt is very pale and hard to match exactly and as the shirt is to be worn over trousers it needs to be long. Some Muslim clothing stores sold shirts in suitable colors but they are either out of stock or too short to be worn with pants. Here are the options I found:
Shukr's Cotton Light Work Shirt in Oceania US $49.95 is a different blue but has the unfussy shirt style of Rayyan's shirt
For a closer color match have a look at
Shukr's Laced Tunic in Paster Blue US $48.95 but you'll have to sacrifice the clean lines of the original for this more ethnic design
This top has contrasting embroidery so is quite different from the original shirt we are copying but it is a close color match to Rayyan's and is cheaper
It is Desert Store's Tunic 06 US $34.99 and although the contrasting embroidery is quite distinct from the color of the shirt it would go nicely with the khaki pants Rayyan wears