What surah in the Qur'an tells the prophet Muhammad (s) to publicly start proclaiming Islam?

Question

As we all know at the beginning of the Revelation, it was a secret, and only select few knew about the Message. The question is what is the name of the chapter or Surah, that Allah ordered Muhammad (s) to start proclaiming Islam publicly?

Answer 1

It was Surah no. 74 verse no.1-3 that ordered the Holy Prophet to start preaching Islam.

And later it was Surah 26 verse 214 in which the Prophet was asked to broaden his teachings a little to some more part of the community.

Then came the revelation of surah 15 verse 94 (something like this) that ordered him to preach openly his message.  

Answer 2

74:1 O THOU [in thy solitude] enfolded!

74:2 Arise and warn!

74:3 And thy Sustainer's greatness glorify!