Reference for Ketubot 145:5

ואי אתמר בהא בהא קאמר שמואל אבל בהך אימא מודי ליה לרב צריכא

And if it had been stated in the former case, it might have been assumed that Shmuel stated his opinion in that case only but that in the latter he agrees with Rav. [Hence both were] required.

Jastrow