Following the Imaam in sajdah even if one did not complete "Rabbana Wa la kal hamd"

When the Imam says Sami Allah Huli man Hamidah, I try to say Rabbana Wa la kalhamd. But before I finish saying it, the imam says Allah Hu Akbar and goes into sajdah before I have finished saying Rabbana Wa la kalhamd. In this case, should I:

Stop saying Rabbana Wa la kal hamd as soon as the Imaam says the takbir and goes into sajdah?

Continue to say Rabbana Wa la kalhamd while following the imam into sajdah?

Do not say Rabbana Wa la kalhamd at all knowing that I won't finish it in time before the imam goes into sajdah.