ANSWER:

What you have heard is absolutely baseless and fabricated and would nullify your fast. 

Once the athan is called, the very first word of ‘Allahuakbar’ is said, you must stop eating, drinking (unless the drink is in your hand then you can finish drinking it) and having intercourse with your spouse immediately. If you continue then your fast is void. 

Would like to clarify if i have heard the fajar azan do i have to stop anything that nullifies the fast?If say one has intercourse with his wife in quick time after the azan ,is his fast still valid? I learnt that the limit to stop sahur is when iqamah is heard or 40 verses of the quran approximately. Appreciate your advise.