The scholar ibn Rushd al-Qurtubi mentions in his book Bidaayatul Mujtahid wa Nihaayatul Muqtasid regarding prostrating when reading a verse of the Qur'aan that signifies prostration:

So as for the ruling of Sujud Tilaawah then Aboo Haneefah and his companions said it is Waajib and Maalik and Shaafi'ee [and Ahmad] said it is sunnah and not Waajib.

And the reason for their difference is that they differed regarding the verses and reports, do they come in the form of an obligation or not? For an example the statement of Allah the Most High:

إِذَا تُتْلَى عَلَيْهِمْ آيَاتُ الرَّحْمَن خَرُّوا سُجَّدًا وَبُكِيًّا

[and] whenever the messages of the Most Gracious were conveyed unto them, they would fall down [before Him], prostrating themselves and weeping.

Does this verse show an obligation or does it show that it is only a sunnah?

Aboo Haneefah understood the verse on its face value that is Waajib. But as for Maalik and Shaafi'ee [and Ahmad] then they followed the understanding of the companions regarding this verse since they are the ones that understood Islaam the best.

It was reported that 'Umar bin Khattaab read surah Sajdah on a friday, then he got down and made Sajdah Tilaawah and the people made sajdah with him. Then the following friday he read surah Sajdah again and the people got ready to make sujud where upon 'Umar said to the people 'At ease, verily Allah has not made it obligatory upon us except that we do it ourselves.

So they [Maalik, Shaafi'ee and Ahmad] said and this [incident with 'Umar] was infront of the Sahaabah, and there is no difference amongst them that has been reported, and they were the ones that understood Islaam as it was ought to be understood. So since there is no dispute amongst the Sahaabah regarding this then it is firm evidence.

[Bidaayatul Mujtahid wa Nihaayatul Muqtasid, volume one, page 423-424]