As far as your question is concerned that Dispatch of dividend warrant for amount exceeding 1500/-should be sent by registered post and below that amount must be dispatched under certificate of posting - where it is wriiten?
Before answering your question let us understand that what is certificate of posting ?
Certificate of posting was a proof of having sent a cover/letter through that particular post office. Postal stamp was to be affixed on a separate paper duly written the from and to address of the letter. The postal authorities use to put their seal on the paper and give us as a certificate for having posted the cover or letter submitted to them.
It was useful to trace the letter/cover in case of missing.
However, vide letter number 2-4/2008-PO dated 23.02.2011 - postal department has discontinued the Certificate of Posting service.
Now, as far as your question is concerned I have gone through Companies Act 2013 and 1956 and rules thereon and no where I have found such written.