Are you using double-distilled water to make your buffer?   If not, the water you are using may be hard water and calcium phosphate will precipitate.  However, that does not explain the disappearance of the precipitate at 37 deg C.     

Thanks.

But i use double distilled water to make my buffers.

Your buffer recipe is not correct. Please check carefully. In general, sodium phosphate salts have a limited solubility and can precipitate in the cold. Potassium based buffers are better in this respect. 

I have made it from the attached file.

This protocol is a little bit strange since you have to add a lot of NaOH to adjust the pH. However, in the end, you should get a similar buffer as mentioned in the link.

Did you consider that these recipes are concentrates, which need to be diluted 5x or 10x before use? If you cool down the concentrate, some salts will precipitate: Therefore, "Store buffer at room temperature." In addition, for some purpose, NaCl should be omitted: Note: Omit the NaCl to prepare Native Purification Buffer without NaCl.

https://en.wikipedia.org/wiki/Phosphate-buffered_saline

I use 1x Buffer as said in recipe.

Dear Nii, the recipe refers to a 5x buffer. I agree with Michael; your phosphate might be too concentrate to stay in solution, especially when cooled down. The presence of NaCl can make it even less soluble.

I believe the problem is with the high NaCl concentration. On ice, the sodium phosphate solubility is decreased sufficiently to crystallize due to the salting out effect of the high Na+ concentration. The equilibrium NaH2PO4 Na+ + H2PO4- is driven to the left by the high concentration of Na+ coming from the NaCl, according to LeChatelier's Principle.

Are you required to use sodium phosphate as the buffer? There are many other buffers you could use at pH 7.4 that would not have this problem. HEPES would be very good.

I will suggest HEPES or PBS buffer.

Thank you all for your suggestions.

Adam B Shapiro i don't have access to HEPES. is it more efficient than sodium phosphate in Ni-NTA column chromatography?

i have read that Sodium phosphate or phosphate–citrate buffer is recommended for IMAC purification instead of HEPES.(Dermot Walls and Sinéad T. Loughran, Protein Chromatography: Methods and Protocols, Methods in Molecular

Biology, vol. 681,chapter17, Purification of Poly-Histidine-Tagged Proteins, page327)